Washington (CNN) The US and South Korean militaries started a massive joint annual exercise Monday, a drill that has drawn North Korean threats of nuclear retaliation.

North Korea will "turn the stronghold of provocation into a heap of ashes through Korean-style pre-emptive nuclear strike" if the US and South Korea "show the slightest sign of aggression" during the drill, a spokesman for North Korea's military was quoted as saying by the country's state media.

U.S. President George W. Bush includes North Korea in an "axis of evil" with Iran and Iraq, which North Korea brushes off as a "little short of a declaration of war." North Korea reportedly threatened to "wipe out the aggressors." That year, North Korea also threatened to kick out international inspectors who were in the country to monitor its compliance with global nuclear nonproliferation agreements.

After the U.S. pledge to give nuclear defense to South Korea, Pyongyang threatened a "fire shower of nuclear retaliation."

North Korea's military threatened to turn the capital of South Korea into a "sea of fire," according to South Korea's Yonhap News Agency.

North Korea's state-run news agency reported that "the moment of explosion is approaching fast" and promised "merciless" strikes against the United States. "The U.S. had better ponder over the prevailing grave situation," it said. Later that month, Pyongyang launched a long-range rocket that broke apart and fell into the sea. The launch came during preparations for a grand party that celebrated the 100th anniversary of the birth of Kim Il Sung, the founder of North Korea.

Once again, North Korea vowed to be "merciless" in its promised attack on the United States, this time threatening a "sacred war" as it aimed artillery at South Korean media groups. North Korea was mad that South Korean journalists had criticized Pyongyang children's festivals meant to foster allegiance to the Kim family.

In a message to the United States and South Korea, North Korea vowed "miserable destruction" if "your side ignites a war of aggression by staging reckless joint military exercises."

Angered by tougher U.N. sanctions and joint military exercises by the United States and South Korea, the Supreme Command of North Korea's military vowed to put "on highest alert" the country's "rocket units" that are assigned to strike "U.S. imperialist aggressor troops in the U.S. mainland and on Hawaii and Guam and other operational zone in the Pacific." Whether Pyongyang has the will to back up such doomsday talk is a perplexing question, but there is evidence that its know-how -- in terms of uranium enrichment, nuclear testing and missile technology -- is progressing.

North Korea threatens to hit the White House and Pentagon with nuclear weapons. American "imperialists threaten our sovereignty and survival," North Korean officials reportedly said after the country accused the U.S. of increasing hostilities on the border with South Korea. "Our troops will fire our nuclear-armed rockets at the White House and the Pentagon -- the sources of all evil," North Korean Gen. Hwang Pyong-So said, according to The Telegraph.

The FBI said it suspected North Korea was behind a hack of Sony Entertainment, which led executives to initially cancel the theatrical release of "The Interview." The film was a comedy about an American television personality who the CIA asks to assassinate North Korean leader Kim Jong Un. North Korea threatened "merciless" action against the U.S. if the film was released, accusing the U.S. of retaliating for the hack by shutting down North Korea's Internet access. North Korea's National Defense Commission also called U.S. President Barack Obama "reckless" and a "monkey."

August 2015: On August 23, as North Korean negotiators were meeting with their South Korean counterparts over current tensions, a KCTV presenter appeared on air repeating North Korea's ambitions to "destroy the warmongering South Korean puppet military."

August 2015: On August 23, as North Korean negotiators were meeting with their South Korean counterparts over current tensions, a KCTV presenter appeared on air repeating North Korea's ambitions to "destroy the warmongering South Korean puppet military."

August 2015: As forces from the U.S. and South Korea took part in joint military drills. North Korea's state media referred to the exercises, which started on August 17, as "madcap" and issued a stern warning to America: "If the U.S. ignites a war in the end, far from drawing a lesson taught by its bitter defeat in the history, the DPRK will bring an irrevocable disaster and disgrace to it."

August 2015: As forces from the U.S. and South Korea took part in joint military drills. North Korea's state media referred to the exercises, which started on August 17, as "madcap" and issued a stern warning to America: "If the U.S. ignites a war in the end, far from drawing a lesson taught by its bitter defeat in the history, the DPRK will bring an irrevocable disaster and disgrace to it."

September 2015: In a statement, North Korea said its nuclear arsenal was ready for use "at any time."

September 2015: In a statement, North Korea said its nuclear arsenal was ready for use "at any time."

January 2016: North Korea claimed to have successfully tested a thermonuclear weapon, justifying its right to have an H-bomb on the grounds of "self defense."

January 2016: North Korea claimed to have successfully tested a thermonuclear weapon, justifying its right to have an H-bomb on the grounds of "self defense."

March 2016: Following the imposition of strict U.N. sanctions, North Korean leader Kim Jong Un said the country's "nuclear warheads need to be ready for use at any time," the North Korean state news agency KCNA reported.

March 2016: Following the imposition of strict U.N. sanctions, North Korean leader Kim Jong Un said the country's "nuclear warheads need to be ready for use at any time," the North Korean state news agency KCNA reported.

North Korea has a history of using creative language to express loathing for its enemies. Here are some of the regime's more colorful threats against the West.North Korea warned it would make a "preemptive and offensive nuclear strike" in response to joint U.S.-South Korean military exercises . Pyongyang issued a long statement promising that "time will prove how the crime-woven history of the U.S. imperialists who have grown corpulent through aggression and war will come to an end and how the Park Geun Hye group's disgraceful remaining days will meet a miserable doom as it is keen on the confrontation with the fellow countrymen in the north."

North Korean forces are "ready to mount a pre-emptive retaliatory strike at all enemy attack groups involved in Ulchi Freedom Guardian," he said, referring to the exercise by its official name.

The exercise takes place almost exactly one year after North Korean troops shelled South Korean territory and only days after South Korea welcomed the highest-ranking North Korean official defector in decades.

That defection, of Pyongyang's deputy ambassador to the UK, caused North Korea to issue a statement calling the diplomat a criminal, with South Korean officials expressing concerns that North Korea might dispatch assassination squads to eliminate potential defectors abroad.

The annual drill will include 25,000 US troops, the bulk of which are already stationed in Korea, according to a statement by US Forces Korea. The purpose of the exercise is to "enhance Alliance readiness, protect the region and maintain stability on the Korean peninsula," the statement said.

South Korean President Park Geun-Hye responded to Pyongyang's threats by saying, "The North Korean regime has been continuously suppressing its people by its reign of terror while ignoring the livelihood of its people."

Photos: How South Korea defends against nuclear North Photos: How South Korea defends against nuclear North CNN is given exclusive access to a training drill underground. Four members of the 23rd Chemical Battalion, the largest chemical unit in the U.S. military, enter an underground facility once used to store nuclear weapons. Hide Caption 1 of 8 Photos: How South Korea defends against nuclear North Troops from the 23rd Chemical Battalion suit up in protective gear ahead of the drill. Hide Caption 2 of 8 Photos: How South Korea defends against nuclear North The team surveys the underground facility suspected of being used to store nuclear weapons. The 23rd Chemical Battalion is the first line of defense against chemical, biological or nuclear attack from North Korea. Hide Caption 3 of 8 Photos: How South Korea defends against nuclear North A remote-controlled robot is deployed to the suspected site to monitor radiation levels and chemical agents. Hide Caption 4 of 8 Photos: How South Korea defends against nuclear North Suited in "Level B" protective suits, chemical resistant boots and gas masks attached to oxygen tanks on their backs, the team takes no chances. Hide Caption 5 of 8 Photos: How South Korea defends against nuclear North The scenario is staged but based on reality. "We take into account the newest intelligence to tailor our training to ensure we're ready for anything that they could possibly have or use on the battlefield," says Lt. Col. Hilburgh. Hide Caption 6 of 8 Photos: How South Korea defends against nuclear North Members of the battalion survey the underground facility. Many of North Korea's nuclear and chemical facilities are believed to be below ground, out of sight of satellites. Hide Caption 7 of 8 Photos: How South Korea defends against nuclear North Soldiers test samples collected in the drill at a temporary field lab. Hide Caption 8 of 8

She added that the South would "prepare" for any possible North Korean provocations, in comments at a National Security Council meeting on Monday.

North Korea's threats come amid new concerns over the increased sophistication of that country's nuclear and missile programs.

"North Korea's nuclear and missile threats are direct and realistic," Park added.

A US official told CNN earlier this month that the regime's aggressive testing of medium- and long-range missiles -- as well as its nuclear testing -- makes North Korea now a "practical" threat and no longer a "theoretical" threat.

"We obviously watch the situation very closely," Pentagon spokesman Peter Cook told reporters Monday.

"It is an annual exercise. We have seen the rhetoric from North Korea, we have seen that rhetoric in the past, and of course we would urge the North Koreans to do what they can to ease tensions on the Korean Peninsula," Cook said. "That is certainly something we would welcome."

Victor Cha, the Korea chair at the Center for Strategic and International Studies in Washington, told CNN that North Korean leader Kim Jong Un is engaging in "much more aggressive" and "much more provocative" behavior than his predecessor and father, Kim Jong Il.

Photos: Kim Jong Un and North Korea's military Photos: Kim Jong Un and North Korea's military North Korean leader Kim Jong Un meets with North Korea's first female fighter jet pilots in this undated photo released by the country's state media on Monday, June 22. He called the women "heroes of Korea" and "flowers of the sky." Hide Caption 1 of 55 Photos: Kim Jong Un and North Korea's military Kim stands on the snow-covered top of Mount Paektu in North Korea in a photo taken by North Korean newspaper Rodong Sinmun on April 18 and released the next day by South Korean news agency Yonhap. Kim scaled the country's highest mountain, North Korean state-run media reported, arriving at the summit to tell soldiers that the hike provides mental energy more powerful than nuclear weapons. Hide Caption 2 of 55 Photos: Kim Jong Un and North Korea's military Kim Jong Un, center, poses with soldiers on the snow-covered top of Mount Paektu in an April 18 photo released by South Korean news agency Yonhap. Hide Caption 3 of 55 Photos: Kim Jong Un and North Korea's military Kim visits the Kumsusan Palace of the Sun in Pyongyang, North Korea, on April 15 to celebrate the 103rd birth anniversary of his grandfather, North Korean founder Kim Il Sung. Hide Caption 4 of 55 Photos: Kim Jong Un and North Korea's military Kim inspects a drill for seizing an island at an undisclosed location in North Korea in an undated picture released by North Korea's official Korean Central News Agency on February 21. Hide Caption 5 of 55 Photos: Kim Jong Un and North Korea's military Kim speaks during a meeting of the Political Bureau of the Central Committee of the Workers' Party of Korea in Pyongyang, North Korea, in this photo released February 19 by the state-run Korean Central News Agency. Hide Caption 6 of 55 Photos: Kim Jong Un and North Korea's military A picture released by the North Korean Central News Agency shows North Korean leader Kim Jong Un appearing without his cane at an event with military commanders in Pyongyang on Tuesday, November 4. Kim, who recently disappeared from public view for about six weeks, had a cyst removed from his right ankle, a lawmaker told CNN. Hide Caption 7 of 55 Photos: Kim Jong Un and North Korea's military Kim is seen walking with a cane in this image released Thursday, October 30, by the state-run Korean Central News Agency. Hide Caption 8 of 55 Photos: Kim Jong Un and North Korea's military Kim sits in the pilot's seat of a fighter jet during the inspection. Hide Caption 9 of 55 Photos: Kim Jong Un and North Korea's military This undated photo, released Tuesday, October 14, by the KCNA, shows Kim inspecting a housing complex in Pyongyang, North Korea. International speculation about Kim went into overdrive after he failed to attend events on Friday, October 10, the 65th anniversary of the Workers' Party. He hadn't been seen in public since he reportedly attended a concert with his wife on September 3. Hide Caption 10 of 55 Photos: Kim Jong Un and North Korea's military A picture released by the KCNA shows Kim and his wife watching a performance by the Moranbong Band on Wednesday, September 3, in Pyongyang. Hide Caption 11 of 55 Photos: Kim Jong Un and North Korea's military Kim tours a front-line military unit in this image released Wednesday, July 16, by the KCNA. Hide Caption 12 of 55 Photos: Kim Jong Un and North Korea's military Kim poses for a photo as he oversees a tactical rocket-firing drill in June. Hide Caption 13 of 55 Photos: Kim Jong Un and North Korea's military Kim watches a tactical rocket-firing drill in June. Hide Caption 14 of 55 Photos: Kim Jong Un and North Korea's military A North Korean soldier patrols the bank of the Yalu River, which separates the North Korean town of Sinuiju from the Chinese border town of Dandong, on Saturday, April 26. Hide Caption 15 of 55 Photos: Kim Jong Un and North Korea's military In this photo released Thursday, April 24, by the Korean Central News Agency, Kim smiles with female soldiers after inspecting a rocket-launching drill at an undisclosed location. Hide Caption 16 of 55 Photos: Kim Jong Un and North Korea's military A picture released Tuesday, March 18, by the KCNA shows Kim attending a shooting practice at a military academy in Pyongyang. Hide Caption 17 of 55 Photos: Kim Jong Un and North Korea's military A North Korean soldier uses binoculars on Thursday, February 6, to look at South Korea from the border village of Panmunjom, which has separated the two Koreas since the Korean War. Hide Caption 18 of 55 Photos: Kim Jong Un and North Korea's military A North Korean soldier kicks a pole along the banks of the Yalu River on Tuesday, February 4. Hide Caption 19 of 55 Photos: Kim Jong Un and North Korea's military A photo released by the KCNA on Thursday, January 23, shows the North Korean leader inspecting an army unit during a winter drill. Hide Caption 20 of 55 Photos: Kim Jong Un and North Korea's military Kim inspects the command of an army unit in this undated photo released Sunday, January 12, by the KCNA. Hide Caption 21 of 55 Photos: Kim Jong Un and North Korea's military Kim visits an army unit in this undated photo. Hide Caption 22 of 55 Photos: Kim Jong Un and North Korea's military Kim inspects a military factory in this undated picture released by the KCNA in May 2013. Hide Caption 23 of 55 Photos: Kim Jong Un and North Korea's military Kim visits the Ministry of People's Security in 2013 as part of the country's May Day celebrations. Hide Caption 24 of 55 Photos: Kim Jong Un and North Korea's military A North Korean soldier, near Sinuiju, gestures to stop photographers from taking photos in April 2013. Hide Caption 25 of 55 Photos: Kim Jong Un and North Korea's military North Korean soldiers patrol near the Yalu River in April 2013. Hide Caption 26 of 55 Photos: Kim Jong Un and North Korea's military Kim is briefed by his generals in this undated photo. On the wall is a map titled "Plan for the strategic forces to target mainland U.S." Hide Caption 27 of 55 Photos: Kim Jong Un and North Korea's military Kim works during a briefing in this undated photo. Hide Caption 28 of 55 Photos: Kim Jong Un and North Korea's military In this KCNA photo, Kim inspects naval drills at an undisclosed location on North Korea's east coast in March 2013. Hide Caption 29 of 55 Photos: Kim Jong Un and North Korea's military Kim, with North Korean soldiers, makes his way to an observation post in March 2013. Hide Caption 30 of 55 Photos: Kim Jong Un and North Korea's military Kim uses a pair of binoculars to look south from the Jangjae Islet Defense Detachment, near South Korea's Taeyonphyong Island, in March 2013. Hide Caption 31 of 55 Photos: Kim Jong Un and North Korea's military Kim is greeted by a soldier's family as he inspects the Jangjae Islet Defense Detachment in March 2013. Hide Caption 32 of 55 Photos: Kim Jong Un and North Korea's military Kim is surrounded by soldiers during a visit to the Mu Islet Hero Defense Detachment, also near Taeyonphyong Island, in March 2013. Hide Caption 33 of 55 Photos: Kim Jong Un and North Korea's military Kim arrives at Jangjae Islet by boat to meet with soldiers of the Jangjae Islet Defense Detachment in March 2013. Hide Caption 34 of 55 Photos: Kim Jong Un and North Korea's military Soldiers in the North Korean army train at an undisclosed location in March 2013. Hide Caption 35 of 55 Photos: Kim Jong Un and North Korea's military In a photo released by the official North Korean news agency in December 2012, Kim celebrates a rocket's launch with staff from the satellite control center in Pyongyang. Hide Caption 36 of 55 Photos: Kim Jong Un and North Korea's military Kim, center, poses in this undated picture released by North Korea's official news agency in November 2012. Hide Caption 37 of 55 Photos: Kim Jong Un and North Korea's military Kim visits the Rungna People's Pleasure Ground, under construction in Pyongyang, in a photo released in July 2012 by the KCNA. Hide Caption 38 of 55 Photos: Kim Jong Un and North Korea's military A crowd watches as statues of North Korean founder Kim Il Sung and his son Kim Jong Il are unveiled during a ceremony in Pyongyang in April 2012. Hide Caption 39 of 55 Photos: Kim Jong Un and North Korea's military A North Korean soldier stands guard in front of an UNHA III rocket at the Tangachai-ri Space Center in April 2012. Hide Caption 40 of 55 Photos: Kim Jong Un and North Korea's military In April 2012, Pyongyang launched a long-range rocket that broke apart and fell into the sea. Here, the UNHA III rocket is pictured on its launch pad in Tang Chung Ri, North Korea. Hide Caption 41 of 55 Photos: Kim Jong Un and North Korea's military – A closer look at the UNHA III rocket on its launch pad in Tang Chung Ri, North Korea. Hide Caption 42 of 55 Photos: Kim Jong Un and North Korea's military A military vehicle participates in a parade in Pyongyang in April 2012. Hide Caption 43 of 55 Photos: Kim Jong Un and North Korea's military North Korean soldiers relax at the end of an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang in April 2012. Hide Caption 44 of 55 Photos: Kim Jong Un and North Korea's military Kim Jong Un applauds as he watches a military parade in Pyongyang in April 2012. Hide Caption 45 of 55 Photos: Kim Jong Un and North Korea's military A North Korean soldier stands on a balcony in Pyongyang in April 2012. Hide Caption 46 of 55 Photos: Kim Jong Un and North Korea's military North Korean soldiers march during a military parade in Pyongyang in April 2012. Hide Caption 47 of 55 Photos: Kim Jong Un and North Korea's military Soldiers board a bus outside a theater in Pyongyang in April 2012. Hide Caption 48 of 55 Photos: Kim Jong Un and North Korea's military North Korean performers sit below a screen showing images of leader Kim Jong Un in Pyongyang in April 2012. Hide Caption 49 of 55 Photos: Kim Jong Un and North Korea's military North Korean soldiers salute during a military parade in Pyongyang in April 2012. Hide Caption 50 of 55 Photos: Kim Jong Un and North Korea's military North Korean soldiers listen to a speech during an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang in April 2012. Hide Caption 51 of 55 Photos: Kim Jong Un and North Korea's military Members of a North Korean military band gather following an official ceremony at the Kim Il Sung stadium in Pyongyang in April 2012. Hide Caption 52 of 55 Photos: Kim Jong Un and North Korea's military North Korean military personnel watch a performance in Pyongyang in April 2012. Hide Caption 53 of 55 Photos: Kim Jong Un and North Korea's military A North Korean controller is seen along the railway line between the Pyongyang and North Pyongan provinces in April 2012. Hide Caption 54 of 55 Photos: Kim Jong Un and North Korea's military A North Korean military honor guard stands at attention at Pyongyang's airport in May 2001. Hide Caption 55 of 55

"We always see North Korea threatening during US-(South Korea) annual military exercises" he said, adding that North Korea does "often provoke around the time of these exercises."

But Cha noted a few differences this time around, including the fallout from "a very embarrassing high-level defection" and North Korea's desire to try to gain leverage amid the upcoming US presidential elections.

"North Korea has a propensity to try and capture everyone's attention," Cha said.

The US military has also been beefing up its presence in the region, jointly deploying three strategic bombers, the B-52 Stratofortress, B-1B Lancer and B-2 Spirit, to the region for the first time.

When asked if the US was concerned about the increase is hostile rhetoric out of Pyongyang, US State Department spokesman Mark Toner answered, "There is always concern, but I don't think that is going to keep us from moving forward."