WASHINGTON (AFP) - The United States (US) warned on Thursday that the danger from North Korea was rising and that Washington was ready for "any eventuality" after flying two nuclear-capable B-2 stealth bombers over ally South Korea.

With tensions soaring on the divided peninsula, the US defied North Korean threats of retribution and took the rare step of announcing that the state-of-the-art jets flew from the United States for the exercises.

Defence secretary Chuck Hagel, dismissing suggestions that the B-2 mission could aggravate tensions, said the US was committed to "unequivocally defend" South Korea as well as Japan.

"We will be prepared - we have to be prepared - to deal with any eventuality," Mr Hagel told reporters at the Pentagon.

"We must make clear that these provocations by the North are taken by us very seriously and we'll respond to that," Mr Hagel said.

The two B-2s flew 20,800km from Whiteman Air Force Base in Missouri and back home without stopping after demonstrating a precision strike by dropping ordnance on a target range in the South.

The US rarely acknowledges B-2 flights to the Korean peninsula, which remains technically at war. B-2 jets, which dodge anti-aircraft defences, bombed targets in conflicts in Serbia, Afghanistan, Iraq and Libya.

The flight came as part of annual drills between the US and South Korea, which North Korea each year denounces as preparations for war but which have drawn particularly fierce criticism this time.

North Korea under young leader Kim Jong Un in recent months has launched a long-range rocket, tested a third nuclear bomb and threatened destruction to US bases in the region and attacks on the US mainland.

In its latest protest at the military drills, Pyongyang announced on Wednesday that it was severing its military hotline with the South, saying it was no longer needed given that "war may break out any moment".

Despite the strident tone, General Martin Dempsey, the top US military officer, said that North Korea's military movements have been "consistent with historic patterns" during training exercises.

Mr Dempsey, speaking alongside Mr Hagel, said that the decision to send the B-2s had less to do with North Korea's reaction than with that of South Korea and Japan.

"Those exercises are mostly to assure our allies that they can count on us to be prepared and to help them deter conflict," Mr Dempsey said.

US policymakers have expressed concern that South Korea or Japan might seek nuclear weapons of their own in response to North Korean threats, fuelling an arms race in a region where North Korea's ally China is rapidly arming.

Most analysts believe that such a prospect is remote given the repeated US assurances of its "nuclear umbrella" and the deep public opposition to nuclear weapons in Japan, the only nation ever attacked with the apocalyptic bombs.

In the 1970s, South Korea's then strongman Park Chung Hee - father of newly elected President Park Geun Hye - flirted with nuclear weapons when the US administration of Jimmy Carter moved to remove US troops from the South.

The US is cutting defense spending in response to concerns about its debt, but both Mr Hagel and Mr Dempsey played down the costs of the B-2 mission and said they were most concerned with deterring North Korea.

North Korea is likely to respond furiously to the B-2 flights. But Pyongyang has been careful not to allow tensions to affect the Kaesong industrial complex, a joint South-North venture that provides the regime crucial hard currency.

The severed military hotline had been used on a daily basis to organise movements in and out of the zone, which lies 10km into North Korea and was set up during a period of reconciliation.

The North has cut off the hotline before, most recently in March 2009. In that case the line was reconnected less than two weeks later.