SEOUL (AFP) - Two nuclear-capable US B-2 stealth bombers flew what the US military described as "deterrence" missions over South Korea on Thursday, in a move sure to further inflame tensions with the North.

The two planes, flying out of Whiteman Air Force base in Missouri, flew the 21,000 km round-trip in a "single continuous mission," dropping dummy ordnance on a target range in the South, the US military said in a press release.

"This .... demonstrates the United States' ability to conduct long range, precision strikes quickly and at will," the statement said.

The two bombers were participating in an ongoing South Korea-US joint military exercise that has infuriated North Korea and prompted threats from Pyongyang to unleash an "all out war" backed by nuclear weapons.

The use of stealth fighters will further inflame tensions, given that the North was already incensed by the participation of B-52 bombers in the exercise.

"The B-2 bomber is an important element of America's enduring and robust extended deterrence capability in the Asia-Pacific region," the US statement said.

In a telephone call with his South Korean counterpart Thursday, US Defense Secretary Chuck Hagel had stressed that all US military capability would be extended to the South, including "the nuclear umbrella, conventional strike means and missile defence."