Pitched Battles Erupt in Sarajevo : Bosnia: Rebel Serbs kill 3 French soldiers in deadliest clashes yet with peacekeepers. Paris threatens pullout. 220 U.N. troops now held hostage. Moscow dispatches envoys.

The Serbs were able to seize a U.N. observation post on the bridge in Sarajevo by disguising themselves as French U.N. soldiers with equipment they have stolen in recent days, U.N. officials said.

The French have the largest contingent in the U.N. peacekeeping force in Bosnia-Herzegovina, and their withdrawal could doom the entire mission and clear the way for all-out war.

The killing of the soldiers--the deadliest direct combat between peacekeepers and Serbs in three years of war--came just hours after French President Jacques Chirac threatened to pull out his troops unless they are given better protection.

BELGRADE, Yugoslavia — Bosnian Serb rebels continued to hold more than 200 U.N. soldiers hostage Saturday, and three French peacekeepers were killed after pitched battles with Serbs who captured a U.N.-held bridge near the center of the Bosnian capital, Sarajevo.

In a flurry of diplomatic efforts, Russia dispatched senior officials to the Balkans on Saturday, and NATO ambassadors, in an emergency session, demanded the immediate release of U.N. hostages.

After cracking down on defiant Serbs by bombing their Bosnian headquarters in Pale last week, the United Nations and the North Atlantic Treaty Organization suddenly find themselves in a tense standoff with the separatists, who say their hostages will die if additional air strikes are launched.

With international resolve crumbling, the world's diplomats were frantically seeking a way out. But the choice seemed to lie between sacrificing the lives of peacekeepers in a hardened military response or giving in to an army that has consistently flouted international conventions.

"What is being threatened here is not just the U.N. soldiers on the ground but the ability of the international community to perform . . . peacekeeping operations in the face of a hostile ground force," U.S. Defense Secretary William J. Perry said Saturday.

Perry held two hours of talks in Naples, Italy, on Saturday with NATO Adm. Leighton Smith, who managed the air strikes that prompted the hostage-taking.

Perry had been scheduled to return to Washington on Saturday night but diverted his military aircraft to London for a hastily arranged consultation with defense ministers Malcolm Rifkind of Britain and Volker Ruehe of Germany.

U.N. officials in Sarajevo said 220 U.N. soldiers and unarmed military observers are being held by the Serbs, with 93 being used as human shields. Some have been chained to bridges and buildings around potential NATO targets.

"They are still being handcuffed and basically treated like animals," U.N. spokesman Alexander Ivanko said Saturday night from Sarajevo.

Journalists in Pale reported seeing some hostages blindfolded and being driven around in their U.N. vehicles. Access to them was granted only to Bosnian Serb television.

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The fighting near downtown Sarajevo began early Saturday when Serbs masqueraded as U.N. personnel--speaking French and going so far as to don blue helmets and U.N. flak jackets--and simply relieved the French of the Vrbanja Bridge observation post near Sarajevo's Parliament building.

Realizing they had been duped, a French infantry platoon backed by light tanks and armored personnel carriers stormed back and recaptured one end of the bridge. The Serbs, firing from nearby high-rise buildings they use for sniper positions, held on to the other end.