The allegations that military leaders used sarin on their own men were made in a programme broadcast by the television news station, CNN, and in a report by Time magazine.

US soldiers are hosed down after chemical warfare exercises

They quote off-the-record remarks by a former chairman of the Joint Chiefs of Staff, Admiral Thomas Moorer.

He is reported as saying that ex-President Richard Nixon and the CIA had approved the use of sarin in a raid aimed at killing a group of men who had deserted in Laos in 1970. Officers who took part in the operation were also interviewed.

Sarin was the agent used in the March 1995 attack on the Tokyo subway by the Aum Shinri Kyo doomsday cult.Twelve people died and about 6,000 others were made sick.

Cohen's pledge

US Defence Secretary William Cohen said he had ordered an inquiry although he was not aware of any information to support the allegations.

He said: "I have asked the service secretaries and those acting in their place to conduct an investigation.

"I have seen no such information to support (the charges) but it's always possible and so we'll look at it.

"I will follow up on the information that comes from the service secretaries."

Senior Vietnam veterans are pouring scorn on the allegations.

Retired General William Westmoreland said: "We used some tear gas for some operations to overcome the enemy resistance, but not sarin nerve gas."

Melvin Laird, defence secretary during the conflict, said that a small amount of nerve gas was shipped to the war zone in 1967, but that it was never used as far as he knew.