American forces have heard Osama Bin Laden giving orders over a short-range radio in the Tora Bora mountains in eastern Afghanistan over the past week, a US official says.

The intercepted radio messages helped persuade US army officials that Bin Laden was probably in the area.

The official, speaking on condition of anonymity, said it was confirmed "with reasonable certainty" that the voice on the radio was Bin Laden's, after it was compared with several videotapes.

The US blames Bin Laden for the 11 September attacks on the World Trade Center and the Pentagon.

The official would not specify what kind of radio transmission the forces had detected, but he did say it was short-range, indicating that Bin Laden was in Tora Bora in the past week.

US troops have several ways to listen in on such transmissions, including gear carried by special operations forces and instruments on aircraft and satellites.