U.S. B-52 bombers carried out simulated nuclear bombing raids on North Korea as part of ongoing U.S.-South Korean military exercises, Pentagon officials said on Monday.

Pentagon press secretary George Little told reporters that B-52 bombers from Andersen Air Force Base, Guam, conducted a training mission over South Korea March 8 during war games known as Exercise Foal Eagle.

"It's not any secret that we are in the midst of sending a very strong signal that we have a firm commitment to the alliance with our South Korean allies," Little said.

Deputy defense secretary Ashton Carter said during a visit to South Korea on Monday that the bomber flights are part of the U.S. "extended deterrence"—the use of U.S. nuclear forces to deter North Korea, which conducted its third underground nuclear test Feb. 12.

"I should note the presence of strategic bombers taking place in flight training in the Korea peninsula area in particular, for example, but this is routine. There will be a B-52 flight tomorrow," Carter said in Seoul.

The bomber flights are part of increased training efforts that seek to show U.S. resolve in protecting South Korea, Little said.

The B-52 flights are part of the U.S. Pacific Command program called Continuous Bomber Presence.

Little said the Guam base has been used since 2004 for strategic bomber rotational deployments. "The B-52 Stratofortress can perform a variety of missions including carrying precision-guided conventional or nuclear ordnance," Little said. "We will continue to fly these training missions as part of our ongoing actions to enhance our strategic posture in the Asia-Pacific region."

It is unusual for the Pentagon to make such overt statements about the use of strategic nuclear forces in Asia Pacific.

The Foal Eagle maneuvers will highlight both nuclear and conventional capabilities of the B-52s, Little said, adding that the flights were routine.

"Despite challenges with fiscal constraints, training opportunities remain important to ensure U.S. and [South Korean] forces are battle-ready and trained to employ airpower to deter aggression, defend South Korea, and defeat any attack against the alliance," he said.

The U.S. military until the early 1990s stored nuclear bombs and missiles in Guam but moved the bombs and missiles to the continental United States.

Two Russian strategic nuclear bombers identified as Tu-95 Bear Hs, were recently intercepted as they circled Guam in what analysts say was saber-rattling on the part of the Russians, who several years ago set up an anti-U.S. alliance with China called the Shanghai Cooperation Organization that includes several Asian and Southwest Asia states.

South Korean news reports last week also stated that U.S. nuclear missile submarines would remain near South Korean waters to provide another sign of U.S. nuclear deterrence.

The combined nuclear and conventional forces exercises began in early March as part of maneuvers called Key Resolve that involved around 13,000 U.S. and South Korean troops. A second round of exercises known as Foal Eagle will extend through the end of April.

South Korean forces remain on heightened alert over concerns that North Korean military forces will attempt a new military provocation, as occurred several years ago when a North Korean submarine sank a South Korean coastal patrol ship and then fired artillery on a South Korean island near the Northern Limit Line delineating North and South Korea waters even with the 38th parallel.

South Korean President Park Geun-hye has said any further military provocation would trigger a South Korean military response, which analysts fear could trigger a new Korea war.

North Korea, angered over a new United Nations resolution condemning its latest underground nuclear test, announced it was pulling out of the 1953 armistice agreement ending hostilities during the Korean War.

Intelligence sources said they do not expect North Korea to conduct a provocation during the military exercises in South Korea when troops and forces are ready to conduct large-scale operations.

However, a future military provocation by Pyongyang could take place in May, after the war games end.

North Korea’s communist government also has stepped up harsh rhetoric with threats to use nuclear missile attacks against both South Korea and the United States.

Propaganda organs recently issued videos on YouTube showing nuclear missile attacks on New York City and U.S. troops in flames from a nuclear strike.

A North Korean general also said recently that the military has missiles equipped with nuclear warheads ready for launch.

The Pentagon on Friday announced it would increase the number of ground-based missile defense interceptors in Alaska and California based on new intelligence indicating North Korea has deployed a road-mobile intercontinental ballistic missile called the KN-08.

The missile has not been flight-tested, but U.S. intelligence agencies believe it is in the process of being fielded and has enough range to reach the United States with a compact nuclear warhead.

Carter, the deputy defense secretary, said he met with senior South Korean officials and is confident the alliance remains strong.

"The key topic of our discussions today was North Korea and its continued pattern of provocative actions that pose a serious threat to the United States and Republic of Korea as well as to regional and global stability," Carter said.

"The United States is working with friends and allies around the world and is employing an integrated response to these unacceptable provocations: The United Nations Security Council resolutions with unprecedentedly strong sanctions, additional unilateral sanctions of great effect, and all together the progressive isolation of North Korea."

The additional missile defense deployments, Carter said, are aimed at bolstering missile defenses as part of "our determination to keep ahead of the progression of the North Korean intercontinental ballistic missile development."

Carter also said he would observe the military exercises.

"In the United States, our commitment to the alliance is part of our Asia-Pacific rebalance, and we will ensure that all the pieces of our defense relationship continue to move forward, and I should say this will occur despite the budgetary pressures in the United States," Carter said.

"The Asia-Pacific rebalance is a priority. It's a historic priority. We have the resources to accomplish it and no matter what happens in the budget debates that go on in the United States, our commitment to the Asia-Pacific rebalance and our commitment to the United States-ROK Alliance will remain firm."