Heavy fighting continues in Afghanistan's Tora Bora region where al-Qaida terrorists remain under U.S. air-strikes as they battle Afghan opposition forces.

Marine General Peter Pace, vice-chairman of the U.S. military's Joint Chiefs of Staff, says al-Qaida forces are fiercely defending what is believed to be their last stronghold.

He tells Pentagon reporters it is a difficult situation. "Al-Qaida have been very aggressive and determined fighters so this is going to be a more difficult situation, especially given the fact that the cave complexes there, which range in the 100's and are interconnecting, will make it much more problematic," he said.

General Pace says it remains unclear whether Osama bin Laden or other top al-Qaida leaders are in this last bastion. "It would be nice if there were leaders there and it would be great if we could kill or capture them but we don't know yet until we uncover the ground," he said.

General Pace acknowledges U.S. forces have yet to take into custody any senior Taleban or al-Qaida personnel. The only detainee currently held at a U.S. Marine base outside the southern Afghan city of Kandahar is John Walker, the American captive who had been fighting with the Taleban.

Still, Pentagon chief spokeswoman Victoria Clarke says the United States has no reason to believe al-Qaida leader Osama bin Laden has managed to flee Afghanistan. "We haven't seen any evidence that leads us to think that," he said.

Nevertheless, a report in the Christian Science Monitor newspaper claims bin Laden escaped from Tora Bora into Pakistan over a week ago. The report quotes a senior al-Qaida operative and Saudi financier interviewed in what was described as a remote Tora Bora area village.

In the meantime, the Pentagon has reported the crash in the Indian Ocean of a U.S. Air Force B-1 bomber.

All four crewmembers have been rescued. The cause of the accident is under investigation.

The plane went down just north of the island of Diego Garcia, a major staging base for U.S. air raids into Afghanistan.

It is the first crash of a fixed-wing U.S. aircraft in the two-month-old anti-terrorist military operation.