NOORDWIJK, Netherlands, May 30 (Reuter) - NATO pledged on Tuesday to give the fragile U.N. force in Bosnia more muscle to prevent any repeat of the hostage crisis that has put the world community on a collision course with the Bosnian Serbs.

German Foreign Minister Klaus Kinkel told reporters the alliance was examining ways of making rapid reaction forces available to the United Nations and the opening of a land corridor to supply Sarajevo, which would be protected by troops.

U.S. Secretary of State Warren Christopher said Washington, which has no troops in Bosnia, would provide "appropriate support" to beef up the U.N. mission of more than 20,000 troops.

Britain and France, the biggest troop contributors in Bosnia, are sending more troops to the region.

A French official told reporters the major powers had agreed at an earlier meeting in The Hague to regroup scattered peacekeepers, protect the "safe areas", demilitarise Sarajevo airport and set up a corridor to secure supply routes to the Bosnian capital and other towns.

Germany's Kinkel and officials from other NATO nations said it was not yet clear how these measures would be implemented and what role the 16-nation alliance would play. They say all countries are opposed to the alternative -- a NATO-led withdrawal from the Balkan quagmire.

Kinkel said negotiations were under way to free the nearly 400 U.N. personnel held hostage by the Bosnian Serbs as a defence against any repeat of last week's NATO air strikes.

"The first aim must be to free the hostages but this does not mean using military force. Negotiations are going on at various levels," Kinkel said.

He refused to give any details but diplomats said Britain and France were pressing Serbian President Slobodan Milosevic to use his influence with Bosnia's rebel Serbs.

NATO Secretary-General Willy Claes told the opening session of the meeting of NATO foreign ministers in the Dutch seaside resort town of Noordwijk that the alliance could not "allow the present situation to continue".

While NATO would back stepped up 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 this was 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.

At the same time, the meeting of the five-nation Contact Group in The Hague agreed to pursue negotiations to end the conflict, once again laying their hopes at Milosevic's door.

The Contact Group members -- Britain, France, the United States, Germany, Russia -- hope they can persuade Milosevic, who in return wants to see the end of international sanctions against Serbia, to recognise Bosnia. This could help dash lingering Bosnian Serb dreams of a Greater Serbia, increasing pressure on the Bosnian Serbs to strike a deal.

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

Moscow has complained that NATO enlargement would leave it isolated but nevertheless plans to start a new military and political relationship with the alliance on Wednesday.

Russian Foreign Minister Andrei Kozyrev will then meet his NATO counterparts and accept two documents giving Moscow a new political and military relationship -- a process stalled since last December.

Christopher urged NATO to start talks on a wider relationship once the documents were accepted to run in parallel with NATO expansion. "We urge an immediate start to the dialogue on the direction our relationship should take," he said.