SARAJEVO, Bosnia-Herzegovina (AP) _ The new U.N. tactic of confronting the Bosnian Serbs escalated into a deadly gunbattle Saturday between Serbs and French peacekeepers for control of a U.N. observation post near the center of Sarajevo.

Two French peacekeepers and four Serbs were killed in fighting that followed NATO airstrikes and the Serbs' detention of more than 200 U.N. soldiers. Some of the soldiers were chained to likely NATO targets as human shields.

World leaders were faced with the choice of getting tougher or backing off _ and possibly backing out of Bosnia. NATO said it supported a stronger U.N. mission. The U.N. Security Council said it would ``not yield to blackmail'' by the Serbs.

Early Saturday, Serb soldiers disguised as French peacekeepers _ complete with U.N. blue helmets and flak jackets _ infiltrated and took command of the U.N. observation post, located at both ends of a bridge over the Miljacka River.

Hours later, French troops backed by six light tanks and armored personnel carriers recaptured one end of the 50-yard-long bridge.

Serbs fired on the French from nearby buildings, and held on to the other side of the bridge, putting them a step closer to the heart of the government-controlled city.

Serb military spokesman Lt. Col. Milovan Milutinovic said the Serbs attacked because they believed the Bosnian army was planning to take over the position.

One French soldier was killed and 10 were wounded fighting for the observation post. One Serb was killed. Another French peacekeeper was killed by a sniper at a nearby post, bringing to 39 the number killed in the former Yugoslavia.

French President Jacques Chirac demanded a tougher mandate for U.N. peacekeepers and instructed French troops to resist Serb aggression on them ``by any means.''

France sent warships Saturday to the Adriatic coast, where the U.S. aircraft carrier Theodore Roosevelt was earlier dispatched.

In Washington, President Clinton spoke by telephone with Chirac and British Prime Minister John Major about Bosnia, including ways to strengthen the peacekeeping mission, said spokesman Calvin Mitchell.

Specific measures were to be discussed Monday night at a NATO foreign ministers' meeting in the Netherlands, Mitchell said. The meeting was moved up from Tuesday.

At the United Nations, Security Council President Jean-Bernard Merimee said council members expressed ``their determination not to yield to blackmail and they appreciated the robust action taken by some elements of the French contingent.''

Merimee, the French ambassador, said the council also discussed options for restructuring the peacekeeping mission and pointed to the gunbattle as one way the United Nations could get tough.

U.S. Defense Secretary William Perry met in Naples, Italy, with the NATO admiral who coordinated repeated airstrikes Thursday and Friday on ammunition depots near Bosnian Serb headquarters outside Sarajevo.

After meeting later in London with his British and German counterparts, Perry said he would ask Russia to intercede with the Serbs, their traditional allies, to convince them their actions are ``counterproductive.''

Russian Prime Minister Viktor Chernomyrdin said in Moscow that Russia would try to facilitate negotiations with the Serbs. But he criticized the NATO bombing.

``It is no good when they try to solve a problem of armed conflict in Bosnia-Herzegovina by just the same military means, delivering powerful airstrikes,'' he said, according to the ITAR-Tass news agency.

The warplanes struck after the Serbs refused to clear the Sarajevo area of heavy weaponry. But instead of backing down after the NATO show of force, the Serbs shelled five of six U.N. protected safe areas, killing 76 people.

They also detained 232 U.N. soldiers, mostly French, and chained some to likely NATO targets. Serbs warned the peacekeepers could be killed if there were further airstrikes. U.N. spokesman Alexander Ivanko said one of the detained peacekeepers contacted the United Nations on Saturday to say they were being well-treated.

Serbs also mined a vital road used by the United Nations to and from Sarajevo. With Serb threats to shoot at aircraft and vehicles trying to cross over the airport, the city was effectively sealed off.

The Muslim-led Bosnian government, almost relegated to the status of bystander in the U.N.-Serb confrontation, urged the world to get tough.

``If the international community backs down now, then it backs down forever to blackmail and terrorism,'' said Prime Minister Haris Silajdzic.

In Paris, Chirac said peacekeepers must be given stronger authority to protect themselves, or else his country would withdraw its troops. France has 3,800 soldiers in Bosnia, the largest single contingent. Withdrawal could influence the rest of the 30,000 troops to follow.

``The grave events happening now are the result of the ambiguity surrounding the mandate of the U.N. troops,'' Chirac said in a statement.

In the past, U.N. soldiers have been more likely to stand between the warring sides and try to negotiate their way out of any trouble. But with heavy fighting around Sarajevo during the last two weeks, U.N. officials have opted for a more confrontational strategy.

U.N. officials said they realized the risks entailed but decided that deteriorating conditions left them no choice.

Bosnian Serb leader Radovan Karadzic, already a war crimes suspect, is increasingly isolated and has little choice but to take on both the Bosnian government and the United Nations.

He has lost the support of Serbian President Slobodan Milosevic, who instigated the Serb rebellion that launched Bosnia's war in April 1992.