NOORDWIJK, Netherlands, May 30 (Reuter) - NATO said on Tuesday it was ready to help beef up the vulnerable U.N. force in Bosnia to prevent any repeat of the hostage crisis which has put the international community on a collision course with the Bosnian Serbs.

"We cannot allow the present situation to continue," NATO Secretary-General Willy Claes told a meeting of alliance foreign ministers which opened in this Dutch seasideresort on Tuesday.

While the 16-nation Western alliance would back continued diplomatic efforts to end the three-year war, it would continue to protect towns designated as "safe areas" and peacekeepers with NATO air power if requested by U.N. commanders.

"The alliance is ready to support efforts towards the reinforcement of U.N. forces in former Yugoslavia, with the aim of reducing their vulnerability and strengthening their ability to perform their essential mission," Claes said.

He gave no details. NATO members Britain, France and the United States are sending more forces to the region.

Diplomats said the alliance might help coordinate a "rapid reaction" force suggested by major powers at a meeting in The Hague earlier on Tuesday as a way of helping stranded and lightly-armed peacekeepers if they were threatened.

U.S. officials have said Washington might provide equipment and logistics but would not send troops to join U.N. forces in Bosnia, which already number more than 20,000.

France, Britain and other troop contributors were due to hold a separate meeting here later on Tuesday and NATO's military committee has drawn up a report on what could be done to help U.N. forces.

Since the defiant Bosnian Serbs seized almost 400 U.N. personnel in response to last week's NATO air strikes, it has become clear that the United Nations is unlikely to order further military action for fear of endangering the lives of the hostages.

But a meeting of the five-nation Contact Group in The Hague agreed in the meantime to strengthen U.N. forces rather than pull them out, while pursuing negotiations to end the conflict.

Nevertheless, NATO ministers will get a briefing on military contingency plans for a U.N. pullout, which would involve an alliance force of some 50,000 troops and include U.S. forces for the first time.

"Withdrawal is a last resort," Claes said. "The continued presence of U.N. peace forces...remains of vital importance...The international community cannot afford to succumb to intimidation and blackmail."

The alliance meeting will also discuss another difficult problem in European security -- the question of NATO expansion into Eastern Europe and often difficult relations with Russia.

Moscow has complained that NATO enlargement would leave it isolated but nevertheless plans to start a new military and political relationship with the alliance at a meeting on Wednesday when Russian Foreign Minister Andrei Kozyrev will meet his NATO counterparts.

"This decision...enhances our ability to achieve a common goal of a truly integrated Europe," said U.S. Secretary of State Warren Christopher.

Washington would also propose steps to strengthen NATO's programme of military cooperation with its former Warsaw Pact foes, known as Partnership for Peace (PFP).

The alliance, he said, would maintain "steady progress" in its discussions about enlargement. NATO has not yet said who will be able to join, nor set any dates.

NATO's Claes said the aim was to reassure Russia and include it in European security affairs, not leave it out in the cold.