It did not disclose the exact number of aircraft, all from Holloman Air Force Base in New Mexico, but US officials say one squadron usually has 15 to 24 fighters.

The deployment was partly to "demonstrate the continued US commitment toward fulfilling security responsibility in Korea and throughout the Western Pacific," it added.

Training deployments usually last four months.

General B.B. Bell, the commander of US forces in South Korea, on Tuesday warned of North Korea's second nuclear test some time in the future, following its first test on October 9.

Some 29,000 US troops support 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 aimed at ending the North's nuclear program ended in Beijing in December without a breakthrough.

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