Afghan opposition forces had Taliban troops on the run throughout the north of the country last night - with the help of British soldiers.

Defence Secretary Geoff Hoon confirmed for the first time yesterday that British forces - believed to be the Special Air Service (SAS) - were active on the ground.

Two days after taking the strategically vital city of Mazar-e-Sharif, the Northern Alliance said it had captured its one-time headquarters of Taloqan and other major strongholds.

Though the Taliban denied Taloqan had been overrun, it was clear that the Taliban was in full retreat in the north, possibly to reorganise its forces around Kabul and southern strongholds of its ethnic Pashtun base.

As the Alliance advanced, US aircraft roamed the skies, blasting Taliban positions on the front line about 30 miles north of Kabul and seeking out retreating bands of Taliban fighters.

In London, the Prime Minister's official spokes-man said it had been "a good 48 hours" for the coalition. "The momentum is very much with us," he said. "The Taliban is hurting."

Though aided by the sustained US airstrikes, the Alliance's rapid advance presents the allies with a difficult dilemma, with US President George W Bush warning them to steer clear of Kabul.

In a clear acknowledgement of the dangers of the capital city falling to the Alliance before a deal is brokered on the make-up of a post-Taliban government, President Bush urged the Alliance to head south but stay out of Kabul - about 200 miles from Mazar-e-Sharif.

President Bush made his comments at a news conference on Saturday with Pakistani President Pervez Musharraf, who warned that if the Alliance took Kabul now, the world would witness "the same kind of atrocities " as after the Soviet Union left Afghanistan.

But restraining the Alliance may prove difficult. Its foreign minister Abdullah Abdullah said he could not rule out a possible advance toward Kabul in the next few days.

He also declined to say that his forces would not move into Kabul in the case of political vacuum developing in the capital.

In London, Mr Hoon said he wanted the Alliance to build on the seizure of Mazar-e-Sharif and make progress towards Kabul.

He said: "We want the Alliance to march towards Kabul, to take ground, to deny the Taliban regime and Osama bin Laden space in Afghanistan."

On the presence of British forces within Afghanistan, Mr Hoon said: "I can certainly confirm that there are members of Britain's armed forces on the ground in northern Afghanistan, liaising with the Northern Alliance, providing advice and assistance."

He did not go into details, but it is thought that the UK forces involved are members of the SAS, and that they have been in the country for several weeks.

The Defence Secretary said he believed that bin Laden, prime suspect in the September 11 terrorist attacks, had "material" that could contribute to a nuclear weapon.

Bin Laden has claimed that he has access to both nuclear and chemical weapons and could use them if the US deploys similar weapons against his al Qaida network.

Mr Hoon said: "We are certainly aware that he has some material that could contribute to a nuclear weapon. We are not convinced at this stage that he is capable of producing a nuclear bomb."

Foreign Secretary Jack Straw said the allies were making a "huge effort" to ensure the Alliance did not repeat the massacres carried out the last time it took Mazar-e-Sharif.

This time, it was aware that it was being watched by the world, it had only won the victory with outside help and its traditional allies were committed to the UN vision of a future regime, he said.

"The only reason that the winner has been able to take all in the past is because of the partisan support for the Taliban or the Northern Alliance from countries in the region.

"There is now an international consensus between the countries surrounding Afghanistan about the nature of its future government.