The crossroads at Mazar-e-Sharif makes it a vital communication centre in Afghanistan, a country with little in the way of formal transport infrastructure.

Its loss is a serious blow for the Taleban. The Northern Alliance's drive northwards towards the Uzbek border would enable the opening up of a land bridge into the country, potentially not just for additional US troops and military supplies, but also for much-needed humanitarian aid.

Of course, the Taleban may seek to try to recapture the city, but to do so they would have to regroup their forces, making them especially vulnerable to the decisive factor in the battle so far - US air power.

The fall of Mazar-e-Sharif is also a vital propaganda victory for the Americans.

They want to persuade various tribal groupings to abandon the Taleban, but clearly have little hope of doing so as long as there is no fundamental change in circumstances on the ground.

All this significantly increases the pressure on the Taleban, putting their capacity for command and control to a severe test.

Nuclear deterrent

Not surprisingly then, Osama Bin Laden has chosen this moment to claim that he has access to both chemical and nuclear weapons.

Puzzlingly, though, he says that he would only use them if the Americans use such weapons against him which is, of course, highly unlikely.

Osama Bin Laden's claims will be taken seriously.

His al-Qaeda organisation is believed to have carried out tests using chemical weapons, but experts doubt that he would have access to a fully- fledged nuclear weapon.

What is a possibility is that he could manufacture a so-called dirty bomb, using smuggled radioactive material packed around a conventional high explosive charge.

This spreads the radioactivity over a large area, though Osama Bin Laden's claims are impossible to verify.