Two postal Washington workers are suspected to have died form pulmonary anthrax and two others are seriously ill in the most serious outbreak of the disease so far. If confirmed they will bring the number of bioterror deaths to three.

The US surgeon general, Dr David Satcher, confirmed that the deaths were regarded with suspicion. "It does seem highly probable that those two deaths were related to inhalation anthrax," he said.

America's postmaster general John Potter described the postal workers as victims of terrorism - although the source of the attack is as yet unknown.