U.S. defense officials say the problem of unintended shipments of potentially live anthrax spores over the past decade is likely worse than first believed.

Officials now say it's possible that more than four dozen such shipments were sent to laboratories in the U.S. and at least three to other countries. That's about twice the estimate of last week.

The officials spoke on condition of anonymity because they weren't authorized to discuss specifics by name.

The Pentagon has repeatedly asserted that the mistakes posed no public health hazard.

Details on the extent of the problem are expected to be presented at a news conference Wednesday by Robert Work, the deputy defense secretary.

Last Friday Work ordered a comprehensive review of laboratory procedures associated with killing, or inactivating, live anthrax for shipment.