The notice to leave Karshi-Khanabad air base, known as K2, was given to the US embassy in the Uzbek capital on Friday.

A Pentagon spokesman said the US was "evaluating the note to see exactly what it means".

Uzbekistan has been an ally of the US in Central Asia, but correspondents say relations were strained over the bloody suppression of a protest in May.

Earlier this month, Russia, China and four Central Asian states demanded a timetable for US troop withdrawal from the region, saying military operations in Afghanistan were coming to an end.

Washington's rivals for regional dominance, Russia and China, have made it clear they do not want to see US forces in the region on a permanent basis.

Kyrgyz backing

The Uzbek government requested in its letter that the US terminate all its operations in Uzbekistan, Pentagon spokesman Glenn Flood said.

He said he did not know why the request had been made and the US state department was assessing the note.

We're always thinking ahead. We'll be fine

Donald Rumsfeld

US Secretary of Defense

Strategic role of 'K2' base

The Washington Post reported that the US had been given six months to move aircraft, personnel and military equipment from the base in southern Uzbekistan.

Mr Flood said the request had not come as a complete surprise, as Washington had always been aware that Tashkent could end the arrangement.

But he said the base had been important for US military operations in neighbouring Afghanistan and that those operations were continuing.

The eviction notice came days after US Defence Secretary Donald Rumsfeld returned from a visit to Uzbek neighbours Kyrgyzstan and Tajikistan.

Officials in Kyrgyzstan affirmed that US forces could continue to use Manas air base for as long as the Afghan war requires.

Mr Rumsfeld said he did not believe US operations in Afghanistan would be hurt if Tashkent denied continued use of K2, because there are other options in the region.

Andijan dispute

Flights into the K2 base had been reduced at the request of the Uzbek authorities, after the US criticised the government over events in Andijan.

Earlier this month, the US signalled that it may withhold $22m of aid to Uzbekistan, unless it allows a full inquiry.

Russia and China fear the US presence undermines its influence

There are still disputed versions of exactly what happened on 13 May, when troops fired on a crowd of people.

The government says the violence was the result of an attempt by Islamic militants to seize power, and puts the number of dead at 173.

But leading human rights groups say many hundreds of civilians were killed, with Human Rights Watch describing the incident as a "massacre".

Washington has already withheld $8m of aid to Uzbekistan in protest at President Islam Karimov's record on human rights.