The Pentagon says a 15,000lb bomb has been dropped at the Tora Bora cave complex in Eastern Afghanistan. The bomb was reportedly dropped after a reported sighting of Osama bin Laden.

A US official said the complex should now have been put out of use. He said the caves were being used by substantial al Qaeda forces and senior leadership.

In the south, a US armoured convoy backed by helicopters has advanced on the former Taliban stronghold of Kandahar from a desert airstrip near the city. US defence officials say they believe the fugitive Taliban leader, Mullah Mohammad Omar, is still in the region, which fell to opposition Afghan forces last week.

The American Vice President, Dick Cheney, has insisted that both bin Laden and Mullah Omar be handed over for trial if they are captured.

Britain has meanwhile accepted the task of leading and organising a multinational force for Afghanistan, which the UN Security Council is expected to approve by Friday.

UN officials are concerned, however, that the NATO nations, who are to form the core of the force, may not be ready to deploy troops by 22 December. That is the date set for a new interim government is to take office in Kabul.

In a further development, US marines have occupied the American embassy in Kabul, returning for the first time in twelve years.