WASHINGTON (AFP) - The White House said on Monday that despite days of bellicose rhetoric, North Korea had yet to back up its threats to the United States (US) and South Korea with mass troop mobilisations or movements.

With tensions on the Korean peninsula rising ever higher, Washington reiterated that it took Pyongyang's war talk seriously but also noted that threats and warnings were nothing new from the isolated state.

"Despite the harsh rhetoric we're hearing from Pyongyang, we are not seeing changes to the North Korean military posture, such as large-scale mobilisations and positioning of forces," White House spokesman Jay Carney said.

"We haven't seen action to back up the rhetoric," Mr Carney said, adding, "what that disconnect between the rhetoric and actions means, I'll leave to the analysts to judge."

Washington has warned North Korea that it will take robust efforts to defend its allies in Asia and repeatedly tells Pyongyang to stand down its nuclear programme and that its "unproductive" rhetoric is self-defeating.

Earlier, the US military said it had deployed stealth fighters to South Korea as part of a 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 Agence France-Presse.

In an earlier show of force, Washington last week dispatched two nuclear-capable B-2 stealth bombers from their base in Missouri on a 21km round trip over South Korea.

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.

The crisis will be top of the agenda when South Korean Foreign Minister Yun Byung Se meets US Secretary of State John Kerry at the State Department on Tuesday, officials said.

Mr Kerry will also directly intervene in the crisis in his debut trip to Asia since taking over from Mrs Hillary Clinton, which begins next week and includes stops in South Korea, Japan and China.

"This will be very much front and centre, and particularly in Beijing," State Department spokeswoman Victoria Nuland said.