Bosnian and Croatian government sources said that they were expecting the imminent arrival of an American military delegation led by retired US general John Galvin, the former supreme commander of Nato forces in Europe.
They said that the delegation would help to unite Bosnian Muslim and Croat forces under the terms of a US-brokered agreement creating a Muslim-Croat federation.
However, both the US State Department and the Pentagon have denied any official sanction for such a mission and have suggested that, if reports about it were true, it was a private initiative.
UN military officials in Sarajevo say that the Americans are already helping the Bosnians, and they provided Muslim military commanders with intelligence and aerial photographs of Bosnian Serb troop dispositions around the north-western city of Bihac at about the same time as the Muslims launched their offensive two weeks ago.
UN military sources and a senior British diplomat have also said that they have heard reports that retired American soldiers were helping to train the Bosnian Muslims. Similar allegations have also been made by French officials.
There have been suggestions by some senior UN civilian officials that British and French UN military officers in Bosnia have been spreading misleading reports about covert US activites in Bosnia to undermine Washington's authority in the debate on the former Yugoslavia. Many UN officials fear any attempts to bolster the Muslims will prolong the war and ruin efforts to find a peaceful solution.
Both the British and the French are opposed to any American moves to lift the arms embargo on Bosnia-Herzegovina and have resisted US-led efforts for tougher Nato air strikes against the Bosnian Serbs for violations of UN resolutions.
President Bill Clinton yesterday ordered three US ships and several spotter planes to pull out of the Bosnia embargo operation at midnight tonight despite criticism from his European allies.
The French Foreign Minister, Alain Juppe, telephoned the US Secretary of State, Warren Christopher, to voice dismay at the action. According to the Foreign Ministry, he ``conveyed the French government's deep worries about this unilateral decision, which we can only regret''.
US defence officials said yesterday's order may have little practical effect because, out of 42,000 ships challenged by the allies and 3,200 boarded since the embargo took effect in November 1992, only three were carrying arms for Bosnia.
``They must be getting arms some other way,'' a senior US military official said.
The Nato Secretary-General, Willy Claes, said in Brussels: ``I would like to make clear . . . that Nato will continue to enforce fully and totally all UN Security Council resolutions which form the basis of our involvement in former Yugoslavia.''
At the Bosnian Serb headquarters in Pale, the Bosnian Serb leader, Radovan Karadzic, said the US decision to stop enforcing the embargo revealed Washington's sympathies in the conflict.
``Now it's clear that America has taken a more sincere position. Although this is going to cause a deterioration of the military situation in Bosnia, we welcome the frankness of America,'' he said.
The Foreign Secretary, Douglas Hurd, who earlier said he was worried by the step, said last night he had talked to Mr Christopher and was ``reassured''. He said: ``We have to take this a step at a time and see what happens on the ground but I feel more confident about it than I did this morning. I am pretty sure our troops will remain through the winter and it is important they do so.''
Bosnia's UN ambassador, Muhamed Sacirbey, said the decision was a signal for nations to break the ban and sell weapons to Sarajevo. ``It will encourage countries that may in the past have been reluctant to give us the means to defend ourselves . . . I am hopeful that we will see almost a campaign of civil disobedience,'' he said.
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