"This is a routine deployment," he said, adding that the move had nothing to do with any particular threat. He declined to give further details, including where and for how long the fighters will be deployed.

Holloman Air Force Base in New Mexico, where the fighters are based, said in a Web posting that the fighters left earlier this week for South Korea.

General B.B. Bell, the commander of US forces in South Korea, said Tuesday he believes North Korea will test a second nuclear bomb at some time in the future, following its first test on October 9.

"Should North Korea attack the South in any way, the combined forces command will respond and we will win quickly and we will win decisively," he said.

Bell heads 29,000 US troops in the peninsula, supporting South Korea's 680,000-strong armed forces against any attack from the North's 1.1 million military.

A South Korean foreign ministry official said last week that activity had been detected at Punggye in northeast North Korea, near the site of the first test, but there was no sign of preparations for a second detonation.

The latest round of six-nation talks on the North's nuclear programme ended in Beijing in December without a breakthrough. The US State Department has said the talks could resume as early as this month.

Analysts say North Korea could set off a second nuclear device to strengthen its hand in the prolonged stand-off.

Related Links
Learn about nuclear weapons doctrine and defense at SpaceWar.com