(CNN) In its latest bout of saber rattling, North Korea says it is ready to use nuclear weapons against the United States and other foes if they pursue "their reckless hostile policy" toward Kim Jong Un's regime.

In a statement carried by the North's state-run Korean Central News Agency on Tuesday, an atomic energy official said Pyongyang is improving its nuclear weapons arsenal "in quality and quantity."

"If the U.S. and other hostile forces persistently seek their reckless hostile policy towards the DPRK and behave mischievously, the DPRK is fully ready to cope with them with nuclear weapons any time," the director of the North Korean Atomic Energy Institute said, using an abbreviation of the country's official name, the Democratic People's Republic of Korea.

North Korea's main nuclear complex at Yongbyon, which includes a uranium enrichment plant and a plutonium production reactor, is operating normally, the official told the news agency.

North says weapons are for self-defense

Notorious for issuing alarming and attention-grabbing statements, Pyongyang has repeatedly threatened to use nuclear weapons against the United States. But strong doubts remain over whether it has the missile technology to target the U.S. mainland.

In an indication it wants to advance its missile capabilities, North Korea said Monday it was planning more satellite launches. Prohibited by U.N. Security Council resolutions, such launches are widely seen as a way of testing ballistic missile technology.

Kim's regime didn't say when the next launch would take place, but observers have speculated that it could launch a long-range rocket carrying a satellite in October around the 70th anniversary of North Korea's ruling party.

The atomic energy official on Tuesday reiterated the North Korean stance that its nuclear weapons program is a self-defense measure "in the face of the U.S. extreme hostile policy and nuclear threats towards it."

U.S. stresses missile defense measures

In an interview with CNN earlier Tuesday, U.S. Ambassador to South Korea Mark Lippert said Washington is constantly working with Seoul "to ensure that other allies in the region as well as the U.S. homeland are protected from threats posed by North Korea."

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've moved, over time, a good deal of missile defense capability to the region," Lippert said before North Korea issued the statement about its nuclear program. "Ground-based interceptors to Alaska, surface combatants to the Western Pacific, a THAAD battery on Guam, another radar in Japan in order to be ready and vigilant for anything the North Koreans may or may not do."

THAAD, which stands for Terminal High Altitude Area Defense, is a ballistic missile defense system.

After the North Korean statement, U.S. State Department spokesman John Kirby told a regular news briefing that the United States continues "to call on North Korea to refrain from irresponsible provocations that aggravate regional tensions, and instead focus on fulfilling its international obligations and commitments."

Nuclear complex revamped

The North's announcement that the Yongbyon nuclear complex is up and running comes as little surprise.

During a period of heightened tensions in the region in spring 2013, Pyongyang announced it would revamp and restart the facilities at the site.

In February of this year, U.S. Director of National Intelligence James Clapper said he believed that "North Korea has followed through on its announcement by expanding its Yongbyon enrichment facility and restarting the reactor."

But some North Korea watchers have questioned whether the reactor is operating at full power.

A July article on 38 North , a website that specializes in analysis of North Korea, said satellite imagery suggested the reactor may not have been operating or was only functioning at low power levels.

The report's authors also identified rapid construction at the uranium enrichment plant of a building they theorized could be used "to assemble or store conventional high explosive components of a nuclear weapon."

The North Korean atomic energy official said Tuesday that the Yongbyon facilities were being employed for both economic development and "the building of a nuclear force."

Expert says nuclear arsenal is growing

Kim Jong Un's regime may already have 10 to 15 nuclear weapons, according to David Albright, a former U.N. weapons inspector who now heads the nonprofit Institute for Science and International Security.

In a report in February, Albright predicted that Pyongyang could increase its stockpile to anywhere between 20 and roughly 100 nuclear weapons by 2020

The growing nuclear arsenal poses a serious strategic challenge for the United States.

The U.S. government has repeatedly called on North Korea to commit to denuclearization as a condition of any future negotiations, but Kim's regime has repeatedly dismissed such an idea, demanding to be recognized as a nuclear power.

Pyongyang's statement Tuesday provided few details about its specific grievances with U.S. policy, which it accused of "openly seeking the downfall" of North Korea's "social system."

The combative rhetoric comes just three weeks after North and South Korea reached a deal to dial down tensions in the region that were inflamed by landmine blasts and artillery fire in the Demilitarized Zone that separates the two countries.

South Korean President Park Geun-hye issued a joint statement with EU leaders after a summit in Seoul on Tuesday in which they strongly condemned the North's "continued development of its nuclear and ballistic missile programs."

The leaders urged North Korea to abandon all its nuclear weapons and existing nuclear program in a "complete, verifiable, and irreversible manner," and to "refrain from any further provocation."