Six thousand British troops were last night preparing to head for Afghanistan as Taliban fighters appeared ready to give up Kunduz - their final northern stronghold.

Amid the heaviest bombing raids yet by US B52s, an opposition commander said the Islamic militia had offered to surrender the city - but only if foreign fighters loyal to terrorist mastermind Osama bin Laden fighting alongside the Taliban were guaranteed their safety.

It was not immediately clear whether the opposition had accepted the offer.

Defence Secretary Geoff Hoon said he was confident that British forces had an important role to play and would do the job "safely and securely".

The troops will embark on a stabilisation mission to help rebuild the war-torn country following several weeks of air strikes and fighting between Taliban and opposition forces on the ground.

Downing Street said as more and more towns and cities fell, bin Laden was "on the run and cannot hide from justice for ever".

"I think if bin Laden is sitting in his cave at the moment he will know that his right-hand man has been killed, and the Taliban is collapsing like a pack of cards," the Prime Minister's official spokesman said.

"We have to recognise that he is an elusive character. It may take some time, but he is on the run and he cannot hide from justice for ever, and he will face up to what he has done.

"We have very specific objectives and we will keep on keeping on until they are all met."

Confirming the number of British military personnel on standby, Mr Hoon said a "number of options" were being considered by the Government.

Members of the Special Boat Service (SBS) are already preparing Bagram airbase north of Kabul for humanitarian airlifts.

The Defence Secretary told BBC 1: "This is an international coalition operation and we need to make absolutely sure everyone is agreed on the next stage forward."

He scotched reports that the Northern Alliance did not want British troops in the area, saying leaders had been "encouraging".

"There have been discussions both with the established leadership and some of the local leadership on the ground," said Mr Hoon. "They are being encouraging and very positive.

"There is a recognition that we can help provide technical expertise which will be of enormous assistance to the Northern Alliance as they begin the process we are involved in of rebuilding Afghanistan."

Despite reports of the Northern Alliance being heavy-handed, Mr Hoon said they had behaved "very responsibly" and the country was not likely to suffer "a complete implosion" owing to warring factions.

The conditional surrender offer in Kunduz came after US B52s led a day of strikes on Taliban positions outside the city, unleashing the largest bombs yet on the area.

Flames shot into the air, and massive booms rolled across the valley floor toward the Northern Alliance's own foxholes on opposing ridges.

Avalanches of soil cascaded down the targeted hillsides. Taliban soldiers could be seen running out on the distant ridges, trying to find cover.

l US Secretary of State Colin Powell said yesterday the Northern Alliance had agreed to take part in United Nations-brokered talks about forming a new power-sharing government in Afghanistan.

The head of the alliance, Burhanuddin Rabbani, said on Saturday that his group supported such a conference in principle.

And yesterday it appeared that it might be prepared to drop its insistence that the meeting be held in Kabul. The UN favours a neutral site.

Mr Powell said he hoped that the meeting, organised by the top UN envoy for Afghanistan, Lakhdar Brahimi, would take place in days.