Story highlights North Korea describes the move as as "unpardonable provocation"

Heavy bombers taking part in U.S.-South Korea exercises

Pentagon: Missions show "U.S. commitment to the security of our allies and partners"

B-52s flying from base on Guam

The U.S. Air Force is breaking out some of its heaviest hardware to send a message to North Korea.

A Pentagon spokesman said Monday that B-52 bombers are making flights over South Korea as part of military exercises this month.

"Despite challenges with fiscal constraints, training opportunities remain important to ensure U.S. and ROK (Republic of Korea) forces are battle-ready and trained to employ air power to deter aggression, defend the Republic of Korea and defeat any attack against the alliance," Pentagon spokesman George Little said Monday in a meeting with reporters at the Pentagon.

Little said the eight-engine bombers first flew as part of the annual Foal Eagle training exercises on March 8 and were to fly again over South Korea on Tuesday.

"This mission highlights the extended deterrence and conventional capabilities of the B-52 Stratofortress," Little said Monday.

JUST WATCHED U.S. lawmaker: Kim Jong Un is immature Replay More Videos ... MUST WATCH U.S. lawmaker: Kim Jong Un is immature 04:43

JUST WATCHED Nuclear fears in South Korea Replay More Videos ... MUST WATCH Nuclear fears in South Korea 02:41

JUST WATCHED U.S. to boost missile defense Replay More Videos ... MUST WATCH U.S. to boost missile defense 02:01

JUST WATCHED North Korean video targets White House Replay More Videos ... MUST WATCH North Korean video targets White House 01:53

The bombers are flying out of Andersen Air Force Base on the Pacific island of Guam as part of what the U.S. Pacific Command calls a "continuous bomber presence" in the region. The round trip between Guam and the Korean Peninsula is about 4,000 miles.

"These ... missions are routine and reiterate the U.S. commitment to the security of our allies and partners," Little said.

The B-52 flights come amid spiking tensions between North Korea and the United States after the U.N. Security Council voted to impose tougher sanctions on North Korea following its latest nuclear test last month.

In a slew of angry rhetoric in response to the U.N. vote, North Korea has threatened to carry out a pre-emptive nuclear attack on the United States and South Korea and said it was nullifying the armistice agreement that stopped the Korean War in 1953.

Reacting to the U.S. flights, a spokesman for the North Korean foreign ministry described them as an "unpardonable provocation," the state news agency reported.

"The DPRK is now closely watching the move of B-52 and the hostile forces will never escape its strong military counteraction, should the strategic bomber make such sortie to the peninsula again," KCNA reported.

Seoul said the B-52 flights demonstrate the strength of the U.S.-South Korea alliance.

"As North Korea threatened to attack South Korea with nuclear weapons, the exercise involving B-52s is meaningful as it shows U.S. commitment to provide its nuclear umbrella on the Korean Peninsula," South Korean Defense Ministry spokesman Kim Min-seok said in a briefing, according to a report from the Yonhap news agency.

U.S. officials said they don't believe North Korea is in a position to strike the United States at the moment, but Defense Secretary Chuck Hagel announced plans last week to deploy additional ground-based missile interceptors on the West Coast as part of efforts to enhance the nation's ability to defend itself from attack.

As for the B-52s, they have been in the U.S. arsenal since 1955. Once part of the country's nuclear triad planes, missiles and submarines, the B-52 is now used predominantly as a conventional bomber and as a platform for air-launched cruise missiles. The Air Force said B-52s dropped 40% of all the munitions used by coalition forces during Operation Desert Storm in the early 1990s.