NATO Tells Bosnian Serbs to Pull Back Arms or Face Strikes : Balkans: The rebels surrounding Sarajevo are offered alternative of putting heavy weapons under U.N. control. Their commanders, anticipating the ultimatum, agree to a cease-fire.

The United States and its West European allies Wednesday issued a 10-day ultimatum to Bosnian Serb forces, warning that they face air bombardment unless they withdraw their heavy weapons from the hills surrounding Sarajevo or place the arms under U.N. control.

Anticipating the ultimatum, Bosnian Serb commanders in Sarajevo agreed earlier in the day to a cease-fire and said they were prepared to give the United Nations control over their artillery.

Wednesday’s decision on air strikes, made here by the ambassadors of the 16 North Atlantic Treaty Organization countries, effectively brings the Western powers to the brink of offensive military action in the Balkans for the first time.

“We will use our air power to stop the terrible loss of life and suffering down there,” NATO Secretary General Manfred Woerner told a news conference after the meeting. “We have to stop the killing.”

The decision called for Bosnian Serb forces either to withdraw or to place under U.N. control all heavy weapons within 20 kilometers (12.6 miles) of central Sarajevo. An area around the nearby town of Pale, where the Bosnian Serbs have their headquarters, was exempted from the zone.

U.S. officials say the NATO plan asks--but does not order--the Bosnian government to put its heavy weapons, estimated to number fewer than 10, under U.N. command as well. They estimate that the Bosnian Serbs have at least 100 heavy weapons. The NATO demands come nearly two years after the start of the bitter war in Bosnia-Herzegovina, which has left 250,000 people dead or missing, and follow previous threats by Western democracies that achieved little.

But NATO is now poised to act, a senior U.S. official told reporters in Washington. “NATO is ready to act in Bosnia. We firmly hope it will not be necessary to do so.”

The alliance set a deadline of 10 days from midnight tonight for completion of the withdrawal and specifically listed the weapons involved--including tanks, artillery pieces, mortars, multiple-rocket launchers and antiaircraft weapons.

“Heavy weapons of any kind . . . found within the Sarajevo exclusion zone, unless controlled by (the United Nations) will, along with their direct and essential military support facilities, be subject to NATO air strikes,” the ultimatum stated.

The 10-day delay, the U.S. official said, is to allow U.N. peacekeeping units to take protective measures.

The declaration noted that any such military action would be conducted “in close coordination with” U.N. Secretary General Boutros Boutros-Ghali.

The alliance on Wednesday also accepted Boutros-Ghali’s request to use NATO forces for air strikes in a second instance: against any artillery or mortar positions in or around Sarajevo that fire on civilian targets.

Wednesday’s decision stems directly from the groundswell of public outrage that reverberated throughout the West after the massacre of 68 people last Saturday when a mortar round exploded in a crowded outdoor market in Sarajevo.

In a letter written the day after the Sarajevo massacre, Boutros-Ghali specifically sought permission to order NATO planes into action under such circumstances.

Woerner said that Boutros-Ghali’s approval would be required prior to any punitive air strikes and that NATO would act “in close coordination with the (U.N.) secretary general” in assessing the need to follow through on the ultimatum. About 100 NATO aircraft, mainly stationed in Italy, plus other planes stationed aboard British, French and U.S. aircraft carriers operating in the Adriatic, would be available if the strikes are necessary.

Even with the force of public opinion suddenly mobilized behind them, the ultimatum did not come easily to alliance members.

NATO ambassadors met here in a marathon 12-hour session before overcoming the final concerns of some member states--notably Greece and Canada.

Canada, one of several NATO countries that have ground forces in Bosnia as part of the 12,000-strong U.N. peacekeeping force, reportedly expressed concern that air strikes might trigger Serbian reprisals against its troops. The Canadian soldiers are particularly exposed in Srebrenica in eastern Bosnia.

Greece, the lone Balkan member of NATO and the only member with strong sympathies toward the Serbs, said it would not participate in any air operations around Sarajevo and would not permit NATO aircraft to launch strikes from bases in Greece.

“We are against any Balkan people being involved in this,” Greek Deputy Foreign Minister Georg Papandreou told the Cable News Network on Wednesday.

There was also concern about the political and military risks involved in air strikes.

Slow-moving antitank aircraft would be potential targets for Bosnian Serbs’ surface-to-air missiles, and opinion is sharply divided on whether NATO involvement would curtail the violence or escalate it.

Wednesday’s decision to issue the ultimatum was also complicated by a strongly negative reaction by Russia, a Serbian ally. Moscow has demanded a meeting of the U.N. Security Council to discuss the prospect of air strikes.

As reports leaked out of the NATO meeting Wednesday afternoon that the alliance was moving toward agreement on the air strikes, Bosnian Serb and Muslim military commanders struck a cease-fire deal that included a promise to put heavy artillery on the mountains ringing Sarajevo under U.N. control.

The U.N. would monitor the heavy weapons and deploy troops to sensitive areas under the plan brokered by the U.N. commander in Bosnia, British Lt. Gen. Michael Rose.

But critical details, such as which locations would be covered under the agreement, remained undecided. Rose cautioned reporters not to make too much of the deal.

Neither of the warring parties signed a plan, and no document outlining the agreement was produced. But a summary of the cease-fire agreement provided to reporters indicated that Bosnian Serb army Gen. Manojlo Milovanovic and Gen. Jovan Divjak of the Muslim-led Bosnian army agreed to a cease-fire beginning at noon today.

Deals like this have been brokered before, with military commanders negotiating bargains that political leaders later refused to implement.

Special correspondent Danica Kirka in Vienna and Times staff writer Doyle McManus in Washington contributed to this report.

* NOT DECISIVE MOVE: The allied threat does not mean an end to the war in Bosnia. A6