SARAJEVO, June 1 (Reuter) - Heavy fighting raged in an eastern Bosnian enclave as Western powers called for a strengthening of the U.N. mission in Bosnia and demanded Serb forces free hundreds of peacekeepers being held hostage. Britain, France and the United States said on Wednesday they agreed the peacekeeping operation in Bosnia needed to be reinforced but they appeared no closer to resolving the hostage crisis, which has paralysed the U.N. mission for the past week. The Bosnian Serbs have surrounded or detained 371 U.N. peacekeepers and observers since NATO planes bombed Serb ammunition depots around Bosnian capital Sarajevo a week ago. Britain warned Bosnian Serb leaders they would be held responsible for the safety of the detained peacekeepers but the Serbs have said they want a guarantee of no more air strikes, which NATO has refused to give. Britain is deploying more troops and artillery to strengthen the vulnerable U.N. force and Prime Minister John Major criticised those urging a withdrawal of the

peacekeepers, saying such a move could trigger a wider war in the Balkans. France, a major contributor to the U.N. mission, proposed a rapid-reaction force of about 4,000 that could come to the aid of peacekeepers under threat. French Defence Minister Charles Millon told RTL radio his country had proposed to Britain and other allies that such a force could open a permanent land corridor for aid supplies and reopen Sarajevo's airport. French officials said Britain was expected to participate in the rapid-reaction force, and French military sources said the multinational unit would be equipped with helicopters, artillery, armoured vehicles and mine-clearing equipment. The United States said for the first time it would consider sending ground troops to help U.N. peacekeepers redeploy to safer positions. President Bill Clinton, who had vowed to keep U.S ground troops out of Bosnia, the United States "should be prepared to assist NATO if it decides to meet a request from the United Nations troops for help in a w

ithdrawal or a reconfiguration and a strengthening of its forces". A Pentagon spokesman said later U.S. soldiers could help in any regrouping of peacekeepers but would not take part in France's proposed rapid-reaction force. With the West clearly ruling out waging war against the Serbs, it remained unclear how reinforcing the U.N. contingent would change the situation on the ground for the peacekeepers. The U.N. troops are deployed in a country where they have no peace agreement to enforce and must rely on the consent of the warring factions to carry out their neutral mandate. U.N. Secretary-General Boutros Boutros-Ghali said on Wednesday that if the Security Council wanted to use force in Bosnia it should send multinational troops and not put them under U.N. command. But he favoured a scaled-down peacekeeping force and a revised mandate so it "includes only those tasks which a peacekeeping operation can realistically be expected to carry out", according to a report he submitted to the Council. U.N. official

s expressed alarm over reports of fierce fighting in the mostly Moslem eastern enclave of Gorazde, which is surrounded by Bosnian Serb forces. "It appears that houses in the town are burning and one mortar bomb landed within 20 metres (yards) of the U.N. base in Gorazdetown," said U.N. spokeswoman Major Myriam Souchaki. Bosnian Moslem-led government troops were battling for control of checkpoints which the U.N. was forced to abandon after Bosnian Serbs swooped in and seized 33 British and five Ukrainian peacekeepers on Sunday. Gorazde, a U.N.-designated "safe area", is emerging as the flashpoint most likely to upset U.N. plans to stabilise Bosnia and secure the release of the detained peacekeepers. Britain's senior diplomat in Belgrade, Ivor Roberts, drove to a Serb-held area of Bosnia on Wednesday to warn Bosnian Serb leaders they would be held responsible for the safety of the detained U.N. personnel. Lord David Owen, European Union peace mediator for the former Yugoslavia, announced in London he would res

ign within a month after the failure of three years of negotiations to end bloodshed in the region. The Contact Group -- the United States, Russia, Britain, Germany and France -- was pressing ahead with efforts to further isolate the Bosnian Serb leaders, who have rejected its peace proposals. Robert Frasure, U.S. envoy for the Contact Group, held talks in Belgrade with Serbian President Slobodan Milosevic in a bid to persuade him to recognise Bosnia in return for an easing of economic sanctions on rump Yugoslavia. Frasure was expected to improve the offer to suspend U.N. sanctions, which were imposed on Belgrade three years ago. Milosevic, who has imposed an arms and fuel blockade on the Bosnian Serbs, wants a deal which would make it hard for the United States to reimpose the sanctions later.