The force, recommended by Nato's military committee, would consist of about 1,500 soldiers based in neighbouring Macedonia, near the capital, Skopje.

It would include a helicopter unit to evacuate the observers in an emergency.

Operational plans are not expected to be approved until next week, a Nato official said. They are currently being drawn up by military experts. The ambassadors approved the initial concept.

The force is intended to be able to respond quickly to potential emergencies affecting the 2,000 observers supplied by the Organisation for Security and Co-operation in Europe (OSCE), overseeing the withdrawal of Serb forces from Kosovo.

The Nato official said individual observer teams could encounter local hostility, become trapped in a minefield or require urgent medical evacuation. A wholesale emergency withdrawal of the mission is seen as the least likely task.

France offers bulk of force

Diplomatic sources said France is offering to supply a substantial part of the mission - some 750 troops.

Serbian police are still patrolling in central Kosovo

It also wants to take initial command of the force, although there could be rotations later.

Reports say Britain and Germany will also provide troops. The United States does not wish to send fighting units.

"This force will be mainly European, with contributions from four or five countries," said the French Defence Minister, Alain Richard, in Vienna.

Earlier, France's Nato partners closed ranks with their ally following the arrest of a French army officer on charges of passing Nato secrets to Yugoslavia concerning targets selected for air strikes to compel compliance.

"This is a serious subject but it was very well handled by French authorities, in a very discreet and very positive fashion without risks for the Alliance," Nato Secretary-General Javier Solana said.

Coal mine gun battle

Meanwhile, the Serbian police reported that they had fought a one-hour gun battle with the ethnic Albanian Kosovo Liberation Army, after the KLA injured three Serbs in an ambush near the Belacevac coal mine.

The ambush was the latest in a reported series of raids on the open-pit mine, Kosovo's major source of fuel for generating electricity.