BRUSSELS, June 8 (Reuter) - NATO defence ministers met on Thursday for talks on Bosnia and ways to give alliance backing to a British and French plan for a rapid-reaction force designed to prop up the faltering U.N. mission in former Yugoslavia.

They will also review a study on plans for extending the alliance into eastern Europe by giving membership to its former enemies of the former Soviet-led Warsaw Pact.

The ministers are expected to discuss proposals to provide close air support for the 10,000-strong rapid reaction force and try to clarify issues regarding its command structure and a possible future role in any NATO-led withdrawal operation.

Britain is expected to tell its allies once again that the force, announced last weekend in Paris, is intended to come under existing U.N. mandates.

"We are not going to wage war, that is not the role of the U.N., but it will mean that U.N. forces will have the protection required in certain circumstances," British Defence Minister Malcolm Rifkind told reporters.

Spanish Defence Minister Julian Garcia Vargas said the ministers were unlikely to reach any decisions as talks on the details of the force and its exact links with the current U.N. Protection Force operation were under way in New York.

He also said NATO could not formally rule out further air strikes, similar to those two weeks ago in retaliation for Serb disregard of U.N. orders, that led to the hostages crisis in Bosnia. "NATO cannot renounce air strikes," he said.

Bosnian Serbs have demanded a pledge of no more NATO air strikes as a condition of releasing U.N. personnel still held as hostages.

U.S. Defence Secretary William Perry repeated Washington's position that the Americans would give backing to the force but would not commit ground troops for it. "We strongly support NATO and its support for UNPROFOR," he said.

But several matters concerning the rapid reaction force remain to be clarified and could yet trigger further tension between both the United Nations and NATO and the European allies and the United States, diplomats say.

The United States, which would provide the bulk of planes and attack helicopters providing air cover, is unlikely to approve its use unless the command structure effectively

sidesteps the U.N. civilian structure.

Britain and France have indicated the force will be under the French and British UNPROFOR commanders already in former Yugoslavia, but its members will be wearing national uniforms.

But NATO sources say much attention will also be paid to trying to remove a block on this year's military budget by Turkey as part of long-running tension with Greece in the eastern Mediterranean.