Ahmed Shah Masood is the only person stopping the Taleban from taking complete control of Afghanistan

Kate Clark, BBC Afghanistan correspondent

Analysts argue that if Mr Masood was indeed killed, the way would be opened for the Taleban authorities to capture the parts of Afghanistan not under their control.

The BBC's correspondent in Afghanistan, Kate Clark, says it is almost impossible to estimate how big a blow the assassination of Mr Masood would be to the anti-Taleban forces.

"He is probably the only recognised Afghan leader outside the country, and he is the only opposition leader who is still in place in his home territories and who controls a border to get supplies across.

"He is the only person stopping the Taleban from taking complete control of Afghanistan," Kate Clark says.

Lack of successor

Several analysts argue that the Northern Alliance's central problem is that has no viable successor to Mr Masood.

The opposition to the Taleban is not only the work of one man, no matter how important his skills were. It represents the views of a lot of the Afghan people. One way or another, the opposition will continue

Barnett Rubin, New York University

"It would be very, very difficult to replace him. The obvious man would be Burhanuddin Rabbani, who is the political leader of the alliance. But he doesn't have anything near the credibility or stature of Masood. He just doesn't have the military ability either," says Jason Burke, a British specialist on Afghanistan.

Other leaders, such as Ismail Khan, the former ruler of the city of Herat who recently escaped from a Taleban prison in Kandahar, has enormous support in the west of the country but not more broadly.

Similarly, General Dostum, another leading opposition figure, has only regional backing.

'Weakened but not gone'

Though the situation for the alliance looks bleak, Barnett Rubin of the Centre on International Co-operation at New York University believes opposition to the Taleban is likely to go on because a lot of Afghan's are not supporters of the Taleban.

"The opposition to the Taleban is not only the work of one man, no matter how important his skills were. It represents the views of a lot of the Afghan people. One way or another, the opposition will continue, even if not in the form of military actions by the Northern Alliance."

Mr Rubin concedes that the alliance is militarily weak, and the reported assassination of Mr Masood will only weaken it further.

"But what they have managed to over the past year or so is to hold onto territory in north east Afghanistan through which the supplies flow, and some other leaders have returned to the country and managed to start military activities on a small scale," Mr Rubin told the BBC.

Next moves

The head of the BBC's Persian Section, Baqer Moin, reports from the region that General Fahim, the Northern Alliance's former head of intelligence, has already been appointed as the commander in chief of the opposition forces to the Taleban.

"I think the second move will be to prepare the ground for a major meeting of the Northern Alliance commanders, including General Dostum, Ismail Khan and others, in order to set a strategy for what to do next," Baqer Moin says.