WASHINGTON -- President Bill Clinton has offered to contribute "a substantial portion'' of combat troops to a potential NATO force designed to help evacuate UN peacekeepers in Bosnia, the Pentagon announced Thursday.


The decision represents a sharp turnabout for Clinton, who has refused to join allies in sending forces to Bosnia.


A senior Pentagon official, speaking on condition of anonymity, declined to give a specific number, but said that the overall force would be composed of "a number of brigades.''


A second senior Pentagon official said NATO planning called for about seven or eight such brigades, which would translate into an overall allied force of some 35,000 to 40,000 troops, depending upon whether the brigade was heavily equipped with tanks.


Given that the United States has been asked to contribute more than the majority of such troops, that means at least 20,000 to 25,000 soldiers could be sent to Bosnia.


Nearly 20 countries have contributed a total of about 23,000 peacekeeping troops to Bosnia. With the failure of mediation efforts and the deterioration of the situation in Bosnia, France on Wednesday asked the United Nations and NATO to develop plans to withdraw the peacekeeping troops.


Russia also said Thursday that it was drawing up its own contingency plans for a possible pullout.


"We hope there will be no need to evacuate the peacekeeping forces in the near future. But plans for evacuation are being drafted 'just in case,'" Itar-Tass quoted Nikita Matkovsky, a Russian foreign ministry official, as saying.


He said the need for evacuation may emerge if a UN embargo on arms supplies to Bosnian Moslems is lifted or if any military action, unauthorized by the United Nations, is undertaken.


U.S. Administration officials say they hope there will be no UN pullout, although some Republican leaders have called for it to open the way for air attacks on Bosnian Serbs.


"The president believes it is important that the United States, as a leader of NATO, be ready to assist our allies if their forces are in danger,'' a White House official said.


The official said the peacekeepers have played an important humanitarian role ,and that a pullout could "raise the risk of real human tragedy.''


Nevertheless, the official said, the president is prepared in principle to commit U.S. ground forces to a NATO operation for withdrawal of the peacekeepers.


Any actual deployment, he added, would be subject to final presidential approval and consultation with Congress.


The official said plans for the mission were being drawn up for a range of operations, including a rescue mission under hostile conditions.


Until now, the United States has declined to send ground troops to Bosnia, but the official said Clinton decided to make an exception to show solidarity with allied nations who have put forces on the ground.


The official said any U.S. troops that might be called upon to participate would be withdrawn from the region following the removal of the UN forces.


The Pentagon official said the NATO force "could take care of itself'' under any contingency--meaing it would go in heavily armed and with authorization to defend itself from any type of hostile act.


The official added that the plans for NATO participation in the potential UN withdrawal will be discussed next week in Brussels by the alliance's top military and defense officials, but that should conditions in Bosnia deteriorate, an emergency operation could be put in place in short order.


The Pentagon official declined to speculate where the U.S. forces might be drawn from for such an operation. Talk of a UN pullout heated up when Bosnian Serbs, retaliating for NATO air strikes last month, took peacekeepers hostage and began blocking UN convoys. (AP, Reuters)