Story highlights The planes flew to South Korea and back in one mission, the U.S. says

It says the flights show its ability "to conduct precision strikes quickly"

U.S. and South Korean defense chiefs spoke by phone Wednesday evening

A previous announcement about B-52 flights over South Korea angered the North

The United States said Thursday it flew stealth bombers over South Korea to participate in annual military exercises amid spiking tensions with North Korea.

The B-2 Spirit bombers flew more than 6,500 miles from Whiteman Air Force Base in Missouri to South Korea, dropping inert munitions there as part of the exercises, before returning to the U.S. mainland, the U.S Forces in Korea said in a statement.

The mission by the planes, which can carry both conventional and nuclear weapons, "demonstrates the United States' ability to conduct long range, precision strikes quickly and at will," the statement said.

The U.S. military's announcement earlier this month that it was flying B-52 bombers over South Korea to participate in the routine exercises prompted an angry reaction from the regime of Kim Jong Un, which has unleashed a torrent of threats in the past few weeks.

There was no immediate reaction to the U.S. statement Thursday from the North's state-run Korean Central News Agency.

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"The United States is steadfast in its alliance commitment to the defense of the Republic of Korea, to deterring aggression, and to ensuring peace and stability in the region," the statement said, using South Korea's official name. "The B-2 bomber is an important element of America's enduring and robust extended deterrence capability in the Asia-Pacific region."

The disclosure of the B-2 flights comes a day after North Korea said it was cutting a key military hotline with South Korea , provoking fresh expressions of concern from U.S. officials about Pyongyang's recent rhetoric. There are several hotlines between North and South Korea.

"North Korea is not a paper tiger so it wouldn't be smart to dismiss its provocative behavior as pure bluster," a U.S. official said Wednesday.

U.S. Defense Secretary Chuck Hagel spoke by phone to his South Korean counterpart, Kim Kwan-jin, on Wednesday evening, Pentagon Press Secretary George Little said, noting the "heightened tension on the Korean Peninsula."

The recent saber-rattling from Pyongyang has included threats of pre-emptive nuclear strikes against the United States and South Korea, as well as the declaration that the armistice that stopped the Korean War in 1953 is null and void.

On Tuesday, the North said it planned to place military units tasked with targeting U.S. bases under combat-ready status.

Most observers say North Korea is still years away from having the technology to deliver a nuclear warhead on a missile, but it does have plenty of conventional military firepower, including medium-range ballistic missiles that can carry high explosives for hundreds of miles.

Tensions escalated on the Korean Peninsula after the North carried out a long-range rocket launch in December and an underground nuclear test last month, prompting the U.N. Security Council to step up sanctions on the secretive regime.

Pyongyang has expressed fury over the sanctions and the annual U.S.-South Korean military exercises, which are due to continue until the end of April.

The North has claimed that the exercises are tantamount to threats of nuclear war against it.

Sharp increases in tensions on the Korean Peninsula have taken place during the drills in previous years. The last time the North cut off military communications with the South was during similar exercises in March 2009.