In his news briefing, Pentagon press secretary George Little said B-52 bombers based at Guam's Andersen Air Force Base flew a mission March 8 over South Korea.
Separately, U.S. Deputy Defense Secretary Ash Carter, visiting South Korea, announced a similar B-52 mission set for Tuesday.
The B-52s are described as long-range, strategic heavy bombers that can drop or launch the widest array of weapons in the U.S. inventory. The aircraft can perform a variety of missions, including carrying precision-guided conventional or nuclear ordnance.
"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, adding the B-52 mission is "a stepped-up training effort" to demonstrate American resolve to protect South Korea and to preserve peace and stability on the Korean Peninsula.
The military exercises are taking place against the backdrop of North Korea's increasingly belligerent posturing and threats against the United States and South Korea. The isolated, impoverished Communist country faces tightened U.N. sanctions for its Feb. 12 nuclear test and its earlier long-range missile test.
Last week, Defense Secretary Chuck Hagel announced the United States will add more ground-based ballistic missile interceptors to guard against threats from North Korea and Iran as those two countries step up their efforts to develop long-range missile capabilities. Hagel said the Pentagon will deploy 14 more ground-based at Fort Greely, Alaska, and Vandenberg Air Force Base, Calif., raising the total to 44 from 30. The interceptors will give a nearly a 50 percent boost to U.S. missile defense capability.
Little said the current South Korea-U.S. Foal Eagle mission exercise highlights the extended deterrence and conventional capabilities of the B-52 Stratofortress.
He said the bomber missions are routine and a literal symbol of American resolve in the Pacific. More than 28,000 American forces are based in South Korea.
"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 added.
In South Korea, Carter said during his meetings with Korean officials he reconfirmed a steadfast commitment to the nearly 60-year alliance between the United States and South Korea. He said the United States, in particular, remains committed to extended deterrence offered by the U.S. nuclear umbrella and to ensuring all capabilities remain available to the alliance, he said.
"If the North Koreans think this kind of thing is going to get them anywhere, they're mistaken," Carter said. "The only effect it's having is to bring down upon North Korea the opprobrium of the entire world."
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