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.