SARAJEVO, May 29 (Reuter) - Western countries assembled a task force of combat troops for Bosnia on Monday but also pursued diplomatic efforts to end the crisis caused by the Bosnian Serbs' seizure of almost 400 U.N. hostages.

Serb leaders brandished their defiance, vowing to hold their captives until the "governments (that sent them) and the U.N. give full peace to Serbs."

Miroslav Toholj, information minister of the self-styled Bosnian Serb government, said: "That means the West has to act urgently and unconditionally to stop the war in Bosnia."

The Serbs took the hostages -- from a score of countries but mostly French, British and Ukrainian troops -- in retaliation for two NATO air raids last week.

Alarmed Western governments halted air strikes after the Bosnian Serb Army (BSA) chained some of the men to potential targets as human shields against the return of NATO jets.

U.S. officials said the five Big Powers trying to end the war wanted to strengthen the 22,000-strong U.N. peacekeeping mission on the ground while isolating the Bosnian Serbs politically.

A meeting on Monday of the Contact Group set up by the United States, France, Russia, Britain and Germany will review efforts to coax Serbian President Slobodan Milosevic into recognition of Bosnia in return for eased U.N. sanctions.

The allies believe they can shorten the three-year war if Milosevic is persuaded to cut his former Bosnian Serb proteges further adrift. But the immediate priority remained the safety and rescue of the hostages held under threat of death if NATO jets are unleashed again.

Russian Foreign Minister Andrei Kozyrev said Moscow could not tolerate the "barbaric" treatment U.N. peacekeepers were suffering in Bosnia, Itar-Tass news agency reported.

Kozyrev did not elaborate but he said Moscow hoped to find common ground with other members of the Contact Group.

Alexander Zotov, President Boris Yeltsin's special envoy to former Yugoslavia, was heading for Belgrade as part of efforts to halt the fighting, a Foreign Ministry official said.

The United States and France were each sending 2,000 more troops into the Adriatic and Britain intended to despatch an extra 1,200 with 5,000 more on standby.

The U.N. mission is being reinforced in tandem with high level consideration of its total withdrawal despite the U.N.'s recognition that departure would be catastrophic for Bosnia's civilian population.

An amphibious unit of 2,000 U.S. Marines specialised in swift rescue and combat entered the Adriatic aboard three warships including a helicopter carrier.

The French aircraft carrier Foch was on its way and the United States prepared to offer the French equipment and logistical support even if President Bill Clinton kept his veto on U.S. ground troops for Bosnia.

British Defence Minister Malcolm Rifkind accused the Bosnian Serbs of acting like terrorists and told BBC radio: "We have no intention of launching a war in Bosnia...but the protection of our own forces is a crucial requirement."

Despite the build up, the U.N. forces already in Bosnia remained vulnerable to the Serbs because they are lightly-armed and spread out.

If serious fighting flared, they could suffer serious casualties before help reached them while the West and Russia remain divided on the use of NATO air power, the factor which has always frightened the Serbs in the past.

British Foreign Secretary Douglas Hurd said the scattered U.N. units should be regrouped but even if the Serbs allowed this, the policy risked leaving the mainly-Moslem U.N. "safe areas" defenceless.

France, which led demands for the U.N. force to be properly protected or withdrawn after suffering the U.N.'s heaviest casualties in Bosnia, criticised the NATO air raids for being ill-prepared.