Postal workers are wearing masks and gloves

"Our intention is to have enough vaccine available so that, should the need to use it arise, anyone who needs it can get it," said Dr Jeffrey Koplan, director of the US Centers for Disease Control and Prevention.

Technically eradicated in 1979, the smallpox virus can easily be passed from person to person.

Mailroom contaminated

As US investigators continue efforts to trace the source of letters containing anthrax spores, officials said tiny amounts had been found at a mailroom in the Veterans' Affairs Medical Center in Washington.

But they said the facility's 200 patients were unlikely to be at risk of contamination.

The medical centre had received mail from a Washington postal sorting office that processed an anthrax-laced envelope delivered to the office of Senate Majority Leader Tom Daschle.

Deadly letters

It has also emerged that a New York woman who mysteriously died of inhalation anthrax did handle mail at the hospital where she worked.

Anthrax spreads

17 confirmed infections 4 deaths (1 in Florida, 2 in Washington DC, 1 in New York) 6 ill with inhalation anthrax 7 cases of skin anthrax 13,300 postal workers taking antibiotics as protective measure

Officials are still trying to discover how Kathy Nguyen contracted the disease after initial tests at her home and the hospital proved negative.

But an executive at the Manhattan hospital, Terence O'Brien, was quoted on Sunday as saying that Ms Nguyen had occasionally dealt with the post, although it was not her main job.

Four people have died of anthrax, with another 13 confirmed cases.

President George W Bush says anthrax is being used as a second wave of terrorist attacks, following the plane hijackings on 11 September.

But the Federal Bureau of Investigations last week admitted it had made little progress towards tracing the perpetrators of the anthrax attacks.