General Richard Myers, chairman of the Joint Chiefs of Staff denied the drug was "experimental," rejecting an argument used by a federal judge Monday in granting a preliminary injunction against compulsory vaccinations for US troops.

"This drug that we're using -- the vaccine -- has been around for 40 years," Myers said, adding that it had been approved for use by US Federal authorities.

"The one thing you can do to protect people is this vaccine."

"It will become probably a legal matter to some degree. But from a military standpoint, I think it's very important we have this capability to protect our troops and enable them to do their job."

Judge Emmet Sullivan on Monday granted the injunction pending a full trial on the case, upholding a complaint by six unnamed plantiffs from the Defense Department that the vaccine was an experimental drug "being used for an unapproved purpose."

In his ruling, Sullivan was not persuaded by US government claims that military operations in Iraq and elsewhere would suffer if the military was barred from using the drug.

"The women and men of our armed forces put their lives on the line every day to preserve and safeguard the freedoms that all Americans cherish and enjoy," the opinion said.

"Absent an informed consent or presidential waiver, the United States cannot demand that members of the armed forces also serve as guinea pigs for experimental drugs."

Top Pentagon officials said that they were reviewing their options following the ruling. Secretary of Defense Donald Rumsfeld declined to comment on the legal specifics of the case.

But Assistant Secretary of Defense for Health Affairs William Winkenwerder said Tuesday that the use of the anthrax vaccine, which opponents say may have caused several deaths among US soldiers, was justified.

"This court ruling is not supported by medical science or by medical facts. It challenges the conclusions of America's best medical experts," said Winkenwerder.

"Our paramount concern is for the safety of our service members and the successful accomplishment of their military missions.

"As we learned from the deaths of innocent Americans in the fall of 2001, it doesn't take a Scud missile to kill with anthrax. All it takes is an envelope."

More than 800,000 US troops have received the anthrax vaccine, a program started under the Clinton administration, and retooled under the Bush administration for troops sent to what were deemed high risk areas, including Iraq.

. But hundreds of troops have reportedly refused to be innoculated, leading to some court martials and discharges from the services.

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