Uzbekistan has long had uneasy relations with its southern neighbour

Uzbekistan has a common border on the north-west of Afghanistan and several crucial former Soviet-era military facilities.

This agreement could be a crucial component in the American military action now going on in Afghanistan.

It has been known for some time that US forces are preparing a military base close to the southern border for operations inside neighbouring Afghanistan.

Click here for a look at the positions of Afghanistan's neighbours on the current crisis.

But until now, the Uzbek Government had only allowed US forces in the country for humanitarian purposes and for search and rescue operations.

Now the two countries have signed an agreement to co-operate in the fight against international terrorism.

Uzbekistan has agreed to provide the use of its airspace, military and civilian infrastructure.

Special forces could been sent into action from the base

The agreement stresses that the struggle is against terrorism, not against the Afghan people.

But significantly, it says the visiting troops would be conducting humanitarian operations in the first instance.

The BBC's Peter Greste, who is in Uzbekistan, says the wording clearly holds the door open for military strikes at some point in the future - although the careful phrasing also suggests Uzbekistan is deeply worried that the base will become a target for attacks from its own militant Islamic opposition.

The facility is the closest point on the ground to Afghanistan that the US could use as a launching pad for its operations.

The two countries have also agreed to discuss any threats on Uzbekistan. That suggests the Americans might be prepared to defend the country if it is attacked by Afghanistan.