Washington (CNN) Two US B-1 bombers flew over the Korean peninsula Tuesday in a planned bilateral training mission with Japanese and South Korean F-15 fighter jets, a US military official told CNN.

The US regularly conducts "assurance and deterrence" bomber flights with its military partners in the region, but the drills are often criticized by North Korea as provocative acts.

"These flights with Japan and the Republic of Korea (ROK) demonstrate solidarity between Japan, ROK and the US to defend against provocative and destabilizing actions in the Pacific theater," a US military official told CNN.

The four-engine WC-135 is used to fly through airspace to detect the residue of nuclear blasts. "The aircraft is equipped with external flow-through devices to collect particulates on filter paper and a compressor system for whole air samples collected in holding spheres," the Air Force says. It has two of these jets in the active force.

WC-135 Constant Phoenix – The four-engine WC-135 is used to fly through airspace to detect the residue of nuclear blasts. "The aircraft is equipped with external flow-through devices to collect particulates on filter paper and a compressor system for whole air samples collected in holding spheres," the Air Force says. It has two of these jets in the active force.

The single-engine, single-pilot U-2 is used for high-altitude reconnaissance and surveillance. Flying at altitudes around 70,000 feet, pilots must wear pressure suits like those worn by astronauts. The first U-2 was flown in 1955. The planes were used on missions over the Soviet Union during the Cold War, flying too high to be reached by any adversary. The Air Force has 33 U-2s in its active inventory.

U-2 – The single-engine, single-pilot U-2 is used for high-altitude reconnaissance and surveillance. Flying at altitudes around 70,000 feet, pilots must wear pressure suits like those worn by astronauts. The first U-2 was flown in 1955. The planes were used on missions over the Soviet Union during the Cold War, flying too high to be reached by any adversary. The Air Force has 33 U-2s in its active inventory.

The twin-engine jet trainer, used by the Air Force to prepare pilots for the F-15E Strike Eagle, F-15C Eagle, F-16 Fighting Falcon, B-1B Lancer, A-10 Thunderbolt and F-22 Raptor, first flew in 1959. Almost 550 are in the active force.

T-38 Talon – The twin-engine jet trainer, used by the Air Force to prepare pilots for the F-15E Strike Eagle, F-15C Eagle, F-16 Fighting Falcon, B-1B Lancer, A-10 Thunderbolt and F-22 Raptor, first flew in 1959. Almost 550 are in the active force.

Based on the DC-10 passenger jet, the triple-engine KC-10 is a gas station in the sky with the ability to carry 75 people and 170,000 pounds of cargo. In its six tanks, the KC-10 can carry up to 356,000 pounds of fuel and dispense it while airborne. The Air Force has 59 KC-10s on active duty.

KC-10 Extender – Based on the DC-10 passenger jet, the triple-engine KC-10 is a gas station in the sky with the ability to carry 75 people and 170,000 pounds of cargo. In its six tanks, the KC-10 can carry up to 356,000 pounds of fuel and dispense it while airborne. The Air Force has 59 KC-10s on active duty.

AWACS stands for airborne warning and control system. This four-engine jet, based on a Boeing 707 platform, monitors and manages battle space with its huge rotating radar dome. The planes have a flight crew of four supporting 13 to 19 specialists and controllers giving direction to units around the battle space. The Air Force has 32 E-3s in inventory.

E-3 Sentry AWACS – AWACS stands for airborne warning and control system. This four-engine jet, based on a Boeing 707 platform, monitors and manages battle space with its huge rotating radar dome. The planes have a flight crew of four supporting 13 to 19 specialists and controllers giving direction to units around the battle space. The Air Force has 32 E-3s in inventory.

The Osprey is a tiltrotor aircraft that combines vertical takeoff, hover and landing qualities of a helicopter with the normal flight characteristics of a turboprop aircraft, according to the Air Force. It is used to move troops in and out of operations as well as resupply units in the field. The Air Force has 33 Ospreys in inventory.

CV-22 Osprey – The Osprey is a tiltrotor aircraft that combines vertical takeoff, hover and landing qualities of a helicopter with the normal flight characteristics of a turboprop aircraft, according to the Air Force. It is used to move troops in and out of operations as well as resupply units in the field. The Air Force has 33 Ospreys in inventory.

The C-5, with a wingspan of 222 feet, a length of 247 feet and a height of 65 feet, is the largest plane in the Air Force inventory and one of the largest aircraft in the world. The first versions of the four-engine jet joined the force in 1970. The Air Force expects to have 52 versions of the latest model, the C-5M, in the fleet by 2017.

C-5 Galaxy transport – The C-5, with a wingspan of 222 feet, a length of 247 feet and a height of 65 feet, is the largest plane in the Air Force inventory and one of the largest aircraft in the world. The first versions of the four-engine jet joined the force in 1970. The Air Force expects to have 52 versions of the latest model, the C-5M, in the fleet by 2017.

The four-engine jet joined the Air Force fleet in 1993 with a primary mission of troop and cargo transport. Each plane can carry up to 102 troops or 170,900 pounds of cargo. The Air Force has 187 C-17s on active duty, 12 in the Air National Guard and 14 in the Reserve.

C-17 Globemaster transport – The four-engine jet joined the Air Force fleet in 1993 with a primary mission of troop and cargo transport. Each plane can carry up to 102 troops or 170,900 pounds of cargo. The Air Force has 187 C-17s on active duty, 12 in the Air National Guard and 14 in the Reserve.

The single-engine jet is a mainstay of the Air Force combat fleet. It can perform both air-to-air and air-to-ground missions with its 20mm cannon and ability to carry missiles and bombs on external pods. More than 1,000 F-16s are in the Air Force inventory.

F-16 Fighting Falcon – The single-engine jet is a mainstay of the Air Force combat fleet. It can perform both air-to-air and air-to-ground missions with its 20mm cannon and ability to carry missiles and bombs on external pods. More than 1,000 F-16s are in the Air Force inventory.

The four-engine KC-135 joined the Air Force fleet in 1956 as both a tanker and cargo jet. It can carry up to 200,000 pounds of fuel and 83,000 pounds of cargo and passengers in a deck above the refueling system. More than 400 of the KC-135s are flown by active, Air Guard and Reserve units.

KC-135 Stratotanker – The four-engine KC-135 joined the Air Force fleet in 1956 as both a tanker and cargo jet. It can carry up to 200,000 pounds of fuel and 83,000 pounds of cargo and passengers in a deck above the refueling system. More than 400 of the KC-135s are flown by active, Air Guard and Reserve units.

An A-29 Super Tucano taxis on the flightline during its first arrival, Sept. 26, 2014, at Moody Air Force Base, Georgia. Afghan Air Force pilots trained on the planes that will be used in air-to-ground attack missions in Afghanistan.

A-29 Super Tucano – An A-29 Super Tucano taxis on the flightline during its first arrival, Sept. 26, 2014, at Moody Air Force Base, Georgia. Afghan Air Force pilots trained on the planes that will be used in air-to-ground attack missions in Afghanistan.

An F-15 Eagle takes off from the Andersen Air Force Base, Guam, flight line as two E-3 Sentries are seen in the background.

An F-15 Eagle takes off from the Andersen Air Force Base, Guam, flight line as two E-3 Sentries are seen in the background.

The RC-135U Combat Sent, based at Offutt Air Force Base, Nebraska, provides strategic electronic reconnaissance information to the president, secretary of defense, Department of Defense leaders and theater commanders.

RC-135U – The RC-135U Combat Sent, based at Offutt Air Force Base, Nebraska, provides strategic electronic reconnaissance information to the president, secretary of defense, Department of Defense leaders and theater commanders.

The A-10 Thunderbolt jets, nicknamed "Warthogs," are specially designed for close air support of ground forces. Key to their armaments is a 30mm Gatling gun. The pilot is protected from ground fire by titanium armor, and the plane's fuel cells are self-sealing in case of puncture.

A-10 Thunderbolt – The A-10 Thunderbolt jets, nicknamed "Warthogs," are specially designed for close air support of ground forces. Key to their armaments is a 30mm Gatling gun. The pilot is protected from ground fire by titanium armor, and the plane's fuel cells are self-sealing in case of puncture.

The Strike Eagle is a version of the air superiority fighter adapted to perform ground-strike missions. With a crew of two, the twin-jet can carry and deploy most weapons in the Air Force inventory and operate in any weather. The F-15E was first delivered in 1988. The Air Force lists 219 in its fleet.

F-15E Strike Eagle – The Strike Eagle is a version of the air superiority fighter adapted to perform ground-strike missions. With a crew of two, the twin-jet can carry and deploy most weapons in the Air Force inventory and operate in any weather. The F-15E was first delivered in 1988. The Air Force lists 219 in its fleet.

The F-15 Eagle, the Air Force's main air superiority fighter, became operational in 1975. With a crew of one or two, depending on the model, the twin-engine jets are armed with a 20mm cannon along with Sidewinder or AMRAAM missiles. The Air Force lists 249 F-15 Eagles in its inventory.

F-15 Eagle – The F-15 Eagle, the Air Force's main air superiority fighter, became operational in 1975. With a crew of one or two, depending on the model, the twin-engine jets are armed with a 20mm cannon along with Sidewinder or AMRAAM missiles. The Air Force lists 249 F-15 Eagles in its inventory.

The four-engine jet can fly at 900 mph and carry the largest payload of bombs and missiles in the Air Force inventory. The Air Force has 62 B-1Bs in the fleet.

B-1B Lancer bomber – The four-engine jet can fly at 900 mph and carry the largest payload of bombs and missiles in the Air Force inventory. The Air Force has 62 B-1Bs in the fleet.

The four-engine B-2 heavy bomber has stealth properties that make it hard to detect on radar. Flown by a crew of two, it has an unrefueled range of 6,000 miles and can deliver both conventional and nuclear bombs. Twenty B-2s are in the active inventory. They joined the fleet in 1997.

B-2 Spirit bomber – The four-engine B-2 heavy bomber has stealth properties that make it hard to detect on radar. Flown by a crew of two, it has an unrefueled range of 6,000 miles and can deliver both conventional and nuclear bombs. Twenty B-2s are in the active inventory. They joined the fleet in 1997.

The single-engine F-35A is the Air Force's eventual replacement for the F-16 and the A-10. The supersonic jets, which will be able to conduct air-to-air and air-to-ground attacks, are just beginning to enter the Air Force fleet. Here, an F-35 Lightning II from the 34th Fighter Squadron at Hill Air Force Base, Utah, flys at Royal Air Force Lakenheath, England, April 15, 2017.

The single-engine F-35A is the Air Force's eventual replacement for the F-16 and the A-10. The supersonic jets, which will be able to conduct air-to-air and air-to-ground attacks, are just beginning to enter the Air Force fleet. Here, an F-35 Lightning II from the 34th Fighter Squadron at Hill Air Force Base, Utah, flys at Royal Air Force Lakenheath, England, April 15, 2017.

The twin-engine F-22 stealth fighter, flown by a single pilot and armed with a 20mm cannon, heat-seeking missiles, radar-guided missiles and radar-guided bombs, can perform both air-to-air and air-to-ground missions. The service has 183 of the Raptors, which went operational in 2005.

F-22 Raptor – The twin-engine F-22 stealth fighter, flown by a single pilot and armed with a 20mm cannon, heat-seeking missiles, radar-guided missiles and radar-guided bombs, can perform both air-to-air and air-to-ground missions. The service has 183 of the Raptors, which went operational in 2005.

The AC-130H Spectre and the AC-130U Spooky gunships are designed for close air support, air interdiction and force protection. Armaments on the Spectre include 40mm and 105mm cannons. The Spooky adds a 25mm Gatling gun.

AC-130 gunships – The AC-130H Spectre and the AC-130U Spooky gunships are designed for close air support, air interdiction and force protection. Armaments on the Spectre include 40mm and 105mm cannons. The Spooky adds a 25mm Gatling gun.

A C-130J Super Hercules from the 37th Airlift Squadron flies over Normandy, France, June 3, 2015. First delivered to the Air Force in 1956, the C-130 remains one of the service's most important airlift platforms. More than 140 are still in active units, with more than 180 in the National Guard and a hundred more in the Reserve. The C-130 is powered by four turboprop engines.

C-130 Hercules transport – A C-130J Super Hercules from the 37th Airlift Squadron flies over Normandy, France, June 3, 2015. First delivered to the Air Force in 1956, the C-130 remains one of the service's most important airlift platforms. More than 140 are still in active units, with more than 180 in the National Guard and a hundred more in the Reserve. The C-130 is powered by four turboprop engines.

The first versions of this long-range heavy bomber flew in 1954. A total of 744 were built, the last of those in 1962. The Air Force maintains 58 B-52s in the active force and 18 in the Reserve. A single B-52 can carry 70,000 pounds of mixed munitions, including bombs, missiles and mines. The eight-engine jets have a range of 8,800 miles.

B-52 Stratofortress – The first versions of this long-range heavy bomber flew in 1954. A total of 744 were built, the last of those in 1962. The Air Force maintains 58 B-52s in the active force and 18 in the Reserve. A single B-52 can carry 70,000 pounds of mixed munitions, including bombs, missiles and mines. The eight-engine jets have a range of 8,800 miles.

"We can launch and operate long-range bombers over the Korean peninsula on very short notice, and we hope not to have to do that," Pentagon spokesman US Navy Capt. Jeff Davis said.

The 10-hour mission was carried out by two US Air Force B-1B Lancers assigned to the 9th Expeditionary Bomb Squadron from Dyess Air Force Base, Texas, a US military official told CNN.

The bombers flew from Andersen Air Force Base, Guam, in the vicinity of Kyushu, Japan, the East China Sea, and the Korean peninsula.

They were joined by Japanese and Korean F-15s performing two separate bilateral missions, the official said.

US bombers have maintained a presence in the Pacific since 2004.

US military flexing

Tuesday's bomber flight is just the latest demonstration of US military might in recent months aimed at sending a message to North Korea and its leader Kim Jong Un -- and the sixth B-1 bomber flight conducted by the US since April 1.

Despite a consistent US military presence in his backyard, Kim has defiantly continued to march toward developing a long-range nuclear missile.

Earlier this year, Beijing called on Pyongyang to suspend its nuclear and missile testing while calling on the US to stop military exercises on and near the Korean Peninsula, which North Korea sees as a threat to its sovereignty. But neither the US nor North Korea has showed signs of compromising.

North Korea continues to test missiles at an unprecedented rate, while US warships and aircraft have maintained a consistent presence in the region for months.

Trump's confusing North Korea tweet

While the US conducts bomber flights in the region on a routine basis, Tuesday's joint training mission is particularly noteworthy as it comes just one day after the death of Otto Warmbier -- a 22-year-old American who was detained in North Korea for 17 months.

The North Korean government handed Warmbier over to US authorities in a vegetative state last week and his death has spurred calls for a US response from several lawmakers.

Trump called Warmbier's death at the hands of North Korea a "disgrace," and took to Twitter on Tuesday to offer additional thoughts about China's role in reining in Pyongyang: "While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out. At least I know China tried!"

Multiple US officials told CNN they are checking on what the President meant by "not worked out."

One Trump administration official said bluntly they didn't know what the President was referencing when asked what the tweet meant. Another said there was no meeting earlier in the day that they feel could have spurred this comment or that that tweet foretold some sort of action against North Korea.

White House press secretary Sean Spicer on Tuesday offered few details about how the US plans to respond to Warmbier's death, promising only that the US will "continue to apply economic and political pressure" to change North Korea's behavior.

"The President has spoken very clearly about how he and the first lady and our country feels about the loss of this American," Spicer said.

Spicer said the US is continuing to work with China to apply pressure on North Korea and said the US has seen "positive movement" with China. The US will "continue to work with them and others to put the appropriate pressure on North Korea to change the behavior of this regime," Spicer said.

Asked whether Trump would still consider meeting with Kim Jong Un given the right conditions, Spicer said: "Clearly we're moving further away, not closer, to those conditions being met."

No sitting US president has ever met with the leader of North Korea while in power.