South Korea and the United States are to expand a joint military drill aimed at finding and destroying North Korean weapons of mass destruction (WMD), a news report said Friday.

The training, which simulates the detection and disposal of the North's chemical, biological, radiological and nuclear weapons, was first conducted in 2009.

The exercise will take place during a major joint military drill code-named Key Resolve, Yonhap news agency said, quoting an unidentified military source. This year's Key Resolve exercise is scheduled for March.

"We will expand and strengthen the anti-WMD drill this year," the source was quoted as saying.

The anti-WMD drill involved 150 Americans in 2009 and 350 last year, the official noted, adding the number was expected to rise further this year.

A defence ministry spokesman said he had no information on the report.

Key Resolve is the largest annual military exercise involving the two allies. Last year, 18,000 American troops and 20,000 South Korean soldiers took part. North Korea routinely condemns it as a precursor to invasion.

Experts estimate Pyongyang, which carried out nuclear tests in 2006 and 2009, has enough plutonium to build six or seven atomic weapons, but it is unclear whether the North is yet capable of mounting the warheads on viable missiles.