US experts in weapons of mass destruction are taking part in a military exercise simulating an attack by North Korea on South Korea, the US commander in the South said Thursday.

The communist North has bitterly denounced the US-South Korean exercise as a preparation for a nuclear attack, and vowed to respond to any aggression with its atomic weapons.

The visiting experts are from a team called Task Force Elimination, of the Maryland-based 20th Support Command.

US commander General Walter Sharp said they brought unique WMD expertise to the Key Resolve/Foal Eagle drill, in which 18,000 American troops and 20,000 Koreans are taking part.

"They are here for this exercise and if we ever went to war, they would naturally come also," he told a press conference.

Sharp said troops from both countries have a shared responsibility for locating, securing and eliminating weapons of mass destruction in case of a North Korean attack.

The general said the exercise tests US and South Korean capabilities "to be prepared to fight and win if we had to go to war today". But the US and South Korea insist it is purely defensive.

The North has staged two atomic weapons tests and has thousands of conventionally-armed missiles deployed near the inter-Korean border.

South Korea's defence ministry says its neighbour also has between 2,500-5,000 tons of chemical weapons.

Sharp said the artillery is a focus of the March 8-18 exercise.

"On a tactical level we work very hard every day to make sure we're prepared to deal with the long-range artillery north of the Demilitarised Zone that could bring a lot of destruction to Seoul," he said.

The US has stationed forces in the South since the end of the 1950-53 war which ended only in an armistice, and currently bases 28,500 troops in the country.

He said the North was informed in mid-February of the dates.

"We have been very transparent and open as we always are with when we have these exercises, the fact that they are defensive in nature, and the fact that this is what militaries around the world do," he said.

He declined to give details of the scenario for Key Resolve, a computer-simulated exercise which runs with the Foal Eagle physical deployment of forces.

"What we are training for is all the threats that North Korea can throw at us," Sharp said.

"We watch very closely what North Korea is saying, doing and developing and we adjust our war plans based upon that. We adjust our exercises based on how we think it would attack."

Sharp said the US seeks a diplomatic solution to the standoff over the North's nuclear weapons and urged it to return to six-party nuclear disarmament talks.

The North quit the talks last April. As a condition for returning, it wants US agreement to hold talks on a permanent peace treaty and says tensions over the current exercise show the need for such a pact.