The White House said it was "pleased" that the troops had been ordered back out of the Serbian province.

The deployment came despite an assurance to Vice President Al Gore that Russian troops would not enter Kosovo before Nato.

"As (Russian) Foreign Minister (Igor) Ivanov has said, it was an unfortunate mistake and the troops will be withdrawn immediately," said presidential spokesman Joe Lockhart.

"We're pleased that they've agreed to rectify the situation. The constructive talks towards determining Russia's role in this peacekeeping force continue with (Deputy Secretary of State) Strobe Talbott in Moscow."

US officials were taken by surprise: President Clinton was alerted to the television pictures from Pristina showing the Russian entry just hours after assuring an interviewer that he did not expect Russian forces moving through Serbia to cause any problems.

Washington correspondent Richard Lister: "US officials are shaken"

Asked if there was any speculation on the reasons behind the troop movement, another White House spokesman, Mike Hammer said: "We leave it to the Russian government to determine what happened."

It is not known whether the Russian unit's entry into Kosovo resulted from a misunderstanding, or was ordered by rogue elements in the military.

A hawkish Russian general, General Leonid Ivashov, had threatened to by-pass Nato and send the troops in if it could not agree on a command structure with Nato.

US officials are seeking urgent clarification about who was responsible for giving troops their orders and why, if it was a mistake, they were not stopped earlier.

Crisis talks

Talks have continued through the night in Moscow between Mr Talbott and Russian officials on the role of the Russian presence in Kosovo.

He had left Moscow on Friday after inconclusive talks, but turned his plane around in mid-air and went straight back when he heard about the Russian troop movement.

Igor Ivanov: "Mistake"

The Russians have been demanding independent command of Russian peace keepers in a separately allocated sector of Kosovo.

Mr Ivanov said the subordination of Russian generals to Nato commanders, which the Americans have so far been insisting on, is unacceptable.

Russian units, he said, would not take part in an operation with second class status.

Washington is adamant that they will not be given their own sector of the province to patrol and that they must accept Nato command.