The US military said Wednesday a problem involving mistaken shipments of live anthrax is much worse than previously reported, with samples of the lethal bacteria sent to more than 50 laboratories.

Pentagon officials insisted there was no threat to public health but the latest numbers showed small concentrations of the live anthrax were shipped to 51 labs in 17 states and the capital Washington, as well as to Australia, Canada and a US base in South Korea.

Deputy Defense Secretary Robert Work and senior officials acknowledged the number of labs affected could still rise as an investigation into the problem unfolds.

Although officials played down fears of any serious health danger, the stream of embarrassing details emerging over the past week has raised questions about the Pentagon's competence in managing the deadly pathogen.

At least four batches of anthrax at a US Army lab in Utah which officials had believed were "dead" have turned out to be active, according to Work.

Out of those four batches, which dated back to 2005, samples were sent out to labs across the country and abroad for research work.

The four live batches come from a total of 400 batches or "lots" of anthrax at four military facilities that handle the pathogens.

Defense officials have ordered that all anthrax held at the lab repositories be tested to ensure they were inactive, Work said.

As a precaution, lab workers who may have handled the anthrax vials -- which are packaged in a series of water-tight containers -- were undergoing medical treatment.

But officials sought to play down fears of any serious health risk.

"There are no suspected or confirmed cases of anthrax infection among any workers in any of the labs that have received these samples over the last 10 years," Work said.

"We know of no risk to the general public for these samples," he said.

Work added that the concentration of these samples was "too low to infect the average healthy individual."