THE UNITED States's Nato allies last night refused to back its plan to bomb Serbian positions around Sarajevo to lift the siege on the Bosnian capital and protect humanitarian aid.

After a day of wrangling in Brussels, Nato's

16 nations, notably France and Britain, rejected the US proposal, though they did agree stronger action was in order and promised to study it further.

Malcolm Rifkind, the Secretary of State for Defence, said any such action must be consistent with United Nations guidelines.

Earlier, President Bill Clinton had pledged that the US and its European allies would act together to stop Sarajevo starving or falling to the Bosnian Serbs. He said in Washington that he did not believe the allies would permit either to happen to the city, adding: 'We are working with the allies. We believe we will be able to work through to a common position.'

Britain, France and Spain are concerned that an air operation should not threaten the Bosnian relief effort or provoke retaliation against thousands of troops they have deployed there under the UN flag.

Nato has deployed at least 60 aircraft in Italy for the Bosnian operation, and alliance officials said last week that the mission could start as early as

today.

Radovan Karadzic, the Bosnian Serb leader, warned the Americans not to attack. 'The peace conference would be ruined. There would be disaster and chaos and a long, long war,' he said.

He accused Muslim delegates at the peace talks in Geneva of obstructing the talks in the hope of securing Western air intervention. Russia's special envoy, Vitaly Churkin, supported that view, saying that the US initiative had come 'at a very bad time for the negotiations'.

Diplomats warned yesterday that the talks between Serbs, Croats and Muslims might collapse because of heavy fighting across Bosnia. Serbian forces dealt Sarajevo's defenders a blow by capturing Mount Bjelasnica, a strategically important point overlooking the capital. The Bosnian government said the position had fallen to an artillery and helicopter attack on Sunday. Use of helicopters would violate a UN ban on military flights.

International mediators secured a promise yesterday from Mr Karadzic that his forces would let UN troops take control of the mountain. The Bosnian President, Alija Izetbegovic, had threatened to leave the Geneva talks unless the Serbs withdrew from Mount Bjelasnica.

The Bosnian government is under Western pressure to sign an agreement that would, in effect, partition Bosnia into Serbian, Croatian and Muslim ethnic zones.

Mr Izetbegovic is loath to accept partition and yesterday delayed talks with the mediators, Lord Owen and Thorvald Stoltenberg. He appeared to pin his hopes on the Nato meeting in Brussels to see whether there was a realistic prospect of Western air strikes against the Serbs.

In central Bosnia, the Muslims scored one of their biggest successes in capturing the town of Gornji Vakuf after defeating Bosnian Croat forces. The Muslims last month captured Bugojno, north of Gornji Vakuf, and began expelling local Croats.

The risk of a renewed war in Croatia increased when Serbian gunners bombarded the Maslenica pontoon bridge, a vital link between northern and southern Croatia. Croatian forces captured the area last January in an operation that shattered a UN- brokered ceasefire in Croatia.

The Croats agreed last month to put the bridge and the nearby Zemunik airport under UN supervision, but later stipulated that Serbian forces in the area should give up their heavy weapons. The Serbian response was to shell the bridge, parts of which were sinking last night into the Adriatic waves.