The US military said Monday it had deployed stealth fighters to South Korea as part of an ongoing joint military exercise that has triggered dire North Korean threats of armed retribution.

Two F-22 Raptor fighters arrived in the South on Sunday to participate in the annual "Foal Eagle" exercise that will last until April 30, a spokesman for the US forces in South Korea told AFP.

The jets were reportedly flown out of the US air base in Okinawa, Japan.

Although it is not the first time F-22s have been used in the annual drill, their presence is sure to further infuriate Pyongyang at a time of sharply escalating military tensions.

North Korea has already threatened to strike the US mainland and US bases in the Pacific in response to the participation of nuclear-capable US B-52 and B-2 stealth bombers in this year's "Foal Eagle" exercise.

On Saturday, North Korea declared it had entered a "state of war" with South Korea and warned Seoul and Washington that any provocation would swiftly escalate into an all-out nuclear conflict.

South Korea and the United States have met the near-daily threats from Pyongyang with their own warnings of severe repercussions, fuelling international concern that the situation might spiral out of control.

S Korea pledges 'strong response' to N Korea

South Korean President Park Geun-hye said on Monday there would be strong and swift military response to any North Korean provocation without regard to political consequences, after Pyongyang said at the weekend it was entering a state of war with its rival.
 
"If there is any provocation against South Korea and its people, there should be a strong response in initial combat without any political considerations," Park told the minister of defence and its senior officials at a meeting on Monday.    

N Korea left few clues about tested nuke design

North Korea likely took meticulous steps to conceal any residue from its February nuclear weapon test, fueling suspicions that it is using a new bomb design with highly enriched uranium at its core, The Washington Post reported Sunday.

Citing unnamed US officials and weapons experts, the newspaper said the effects of the February 12 explosion were remarkably well contained, with few radioactive traces escaping into the atmosphere.

The US government anticipated the nuclear test, North Korea's third, and monitored it closely for clues about the composition of the bomb, the report said.

But in the days following the detonation, US and South Korean sensors failed to detect even a trace of the usual radioactive gases in any of the 120 monitoring stations along the border and downwind from the test site, the paper said.

A Japanese aircraft recorded a brief spike of one radioactive isotope, xenon-133, but it was seen as inconclusive, The Post said.

According to the paper, the absence of physical data could suggest a deliberate attempt by North Korea to prevent the release of telltale gases, presumably by burying the test chamber deep underground.

In its first two nuclear tests, North Korea was thought to have used plutonium extracted from a stockpile of fissile material that the country developed in the late 1990s, the report said.

A successful test of a uranium-based bomb would confirm that Pyongyang has achieved a second pathway to nuclear weapons, using its plentiful supply of natural uranium and new enrichment technology, The Post said.

A device using highly enriched uranium would also deepen worries about cooperation between North Korea and Iran, the paper said. Iran has been concentrating on uranium enrichment.

The United States is concerned about last year's agreement between North Korea and Iran pledging technical and scientific cooperation, The Post said.

The pact was signed in Tehran in September at a ceremony attended by Iranian President Mahmoud Ahmadinejad and Fereydoun Abbasi, the head of Iran's nuclear program.