Brief Shelling Interrupts New Sarajevo Cease-Fire : Bosnia: U.S., U.N. officials play down the violation. It still is unclear whether Serbs will heed NATO ultimatum.

The U.N.-negotiated cease-fire between Bosnian Serb and Muslim factions in Sarajevo went into effect as scheduled Thursday, but the truce was shattered briefly just before midnight when two artillery shells crashed into the city, accompanied by a barrage of machine-gun fire.

U.S. and U.N. officials played down the incident, saying it was difficult to tell which of Bosnia-Herzegovina’s warring factions had fired the shells and suggesting that the violation may not have been sufficient to trigger retaliatory air strikes, as NATO has threatened. The machine-gun fire lasted 10 minutes.

Reports from the scene said the artillery shells exploded near the Vrbanja bridge, in a nearly empty section of the city that has not been turned over to U.N. peacekeeping forces. There were no immediate reports of injuries.

Meanwhile, U.S. officials said it still was unclear whether Bosnia’s Serbs planned to heed a NATO ultimatum to pull their heavy weapons--including artillery, mortars, tanks and rockets--out of a 12-mile zone surrounding the city within 10 days.

Although the Serbs have moved a handful of the heavy weapons they have placed near the city, a senior U.S. official said that there had “not been enough change” in the Serbian deployment to tell “whether they (the Serbian leaders) have made a decision on it.”

The Serbs’ actions are being watched closely as an early indication of whether the new North Atlantic Treaty Organization ultimatum is likely to ease tensions in Bosnia or bring about the first allied military intervention there since the fighting began 22 months ago.

Serbian leaders gave conflicting signals. Radovan Karadzic, leader of Bosnia’s Serbs, said that the artillery would be withdrawn soon. But Gen. Manojlo Milovanovic, head of the Bosnian Serb army, called a pullback “out of the question” in the face of the ultimatum.

Meanwhile, reaction in Russia took a more positive turn as the Russian Foreign Ministry moved to ease the effects of criticism by Russian nationalists that NATO’s ultimatum would damage U.S.-Russian relations.

At a briefing in Moscow, a Foreign Ministry spokesman told reporters that NATO’s call for Serbs and Muslims to withdraw their heavy artillery, or place it under U.N. control, is “essentially close to” a Russian proposal to create a demilitarized zone around Sarajevo.

A few hours later, Yuli Vorontsov, Russia’s U.N. ambassador, called for an emergency meeting of the Security Council to revive the Russian proposal, which has been lying dormant for several months. But he said that Moscow would not try to block NATO action.

U.S. officials appeared to be encouraged by the Russian response. President Clinton told reporters that “we have no reason to believe at this point that there’s a serious problem with our going forward.”

The developments came as the Clinton Administration sought to dampen expectations that the air strikes would destroy the Serbs’ artillery and to assure the American public that the NATO action would not mark the start of an escalation of the action in Bosnia.

At a briefing at the Pentagon, Deputy Undersecretary of Defense Walter B. Slocombe told reporters that the air strikes would be designed solely to reduce the amount of shelling on Sarajevo, not to knock out every Serbian artillery emplacement.

He asserted that the United States would not become trapped “on a slippery slope” that might lead to a full-fledged war in Bosnia, as some critics have feared. He called the limited air strikes “the only objective we have at this stage.”

Despite the apparent cooperation, the Administration appeared to be going out of its way to assuage the Russians. Madeleine Albright, U.S. ambassador to the United Nations, met with Vorontsov to discuss the Russian plan and apparently raised no objections.

State Department spokesman Mike McCurry called the session “a good, constructive meeting on the subject of Bosnia and how to proceed at this point.”

At the same time, Clinton’s efforts to telephone Yeltsin to discuss the issue were unsuccessful as the Russian president proved unreachable for the second day in a row.

Clinton characterized Sarajevo as “sort of the Humpty Dumpty” of Bosnia. “If you want everyone to be put back together again--the country--you’ve got to keep Sarajevo from total collapse,” he said during a brief impromptu news conference at the White House.

Under the U.N. ultimatum, Bosnia’s Serbs have until Feb. 21 to either withdraw their heavy weapons--artillery, mortars, rockets and tanks--from the Sarajevo area or turn them over to U.N. peacekeeping forces. If they do not, they will be subject to NATO air bombardment.

NATO warplanes are poised to launch immediate attacks in advance of the Feb. 21 deadline on any Serbian artillery that fires on Sarajevo in violation of the cease-fire arranged on Wednesday. They also are authorized to respond to calls for help from U.N. peacekeeping troops that may be pinned down or under siege.

The Pentagon warned that if the Serbs do not obey the NATO order, the allies will widen their list of targets to go beyond heavy weapons--presumably to include ammunition dumps and supply depots.

While declining to cite specific examples, Slocombe said that the list extends “beyond simply the kind of weaponry which has to be moved out of the zone.” He said the NATO order authorizes attacks on air-defense facilities as well.

At the same time, Slocombe and Brig. Gen. James T. Hill, a planner for the Joint Chiefs of Staff, warned that the military action being planned by NATO is limited in scope and designed only to reduce the shelling of Sarajevo by making it riskier for the Serbs.

“There is no way that anyone can stand up here and guarantee you that we can go after and hit every artillery piece even if we could find it,” Hill told reporters. “This is not an easy task.”

U.S. officials said they now believe that Bosnian Serb forces have between 200 and 300 heavy weapons within the 20-kilometer (12.4-mile) exclusion zone that NATO has established around Sarajevo, rather than only 100, as officials had said on Wednesday.

In related action Thursday:

* Administration officials were reportedly preparing to support the partition of Bosnia among the three warring factions if the opposing sides could agree on details as part of a peace accord. The Administration had previously opposed such a move in the face of the current fighting.

* A report in today’s New York Times said U.S. officials have indicated a willingness to lift sanctions against Serbia on a step-by-step basis.

* Britain suspended its aid convoys in Bosnia, calling the move a “prudent precaution” to prevent reprisals after Wednesday’s action by NATO. The United Nations said that it, too, would withdraw aid workers from some key towns.

* In Geneva, the United Nations agreed to set up a special commission to investigate last weekend’s massacre in a Sarajevo market, ending a threat by Serbs to walk out of the negotiations unless the United Nations brought in independent officials to conduct the inquiry.

* The Pentagon made public new figures showing that the Bosnian Serb army has about 80,000 personnel, about 300 tanks and 200 armored personnel carriers, between 600 and 1,000 artillery pieces, 30 helicopters and 20 fighter aircraft.

Times staff writer Sonni Efron in Moscow contributed to this report.

Past NATO Threats

NATO allies agreed this week to order air strikes against Bosnian Serb positions besieging Sarajevo unless they withdraw their weapons or put them under U.N. control within 10 days. But it is not the first time the North Atlantic Treaty Organization has threatened to use air power in Bosnia-Herzegovina’s civil war. Here are some previous NATO threats:

‘NO-FLY’ ZONE: In March, 1993, the United Nations banned all unauthorized flights over Bosnia and authorized NATO to take “all necessary measures” to ensure compliance. One month later, NATO warplanes began patrolling Bosnian air space under “Operation Deny Flight.” NATO claims the operation “effectively denied the use of air power by all three warring factions as an instrument of war.” But hundreds of violations have been reported, notably by Serbian and Croatian helicopters.

CLOSE AIR SUPPORT: In June, 1993, NATO foreign ministers agreed to provide air cover for U.N. peacekeeping troops in Bosnia. Since then, U.N. commanders under fire have been able to ask U.N. Secretary General Boutros Boutros-Ghali to send in planes to defend them. U.N. military commanders have criticized the process as being too slow and requested that they be given the right to call in attacks. U.N. figures say 33 peacekeepers were killed in war-related incidents through Dec. 31, 1993.

AIR STRIKES: On Aug. 2, 1993, NATO announced it was preparing air strikes against the Bosnian Serbs if they continued their “strangulation” of Sarajevo and other Muslim “safe areas.” A week later, the alliance approved air strikes in support of humanitarian action. In response to those threats, the Serbs pulled troops back from two mountains overlooking the Bosnian capital. But Serbian forces have continued to shell Sarajevo and have regularly blocked humanitarian aid.

TUZLA AND SREBRENICA: At a Jan. 11 NATO summit, President Clinton and other leaders repeated warnings of using air power to break the siege of Sarajevo. They also threatened air strikes if the Serbs refused to allow aid flights to land at Tuzla airport and permit Dutch U.N. troops to replace Canadian soldiers in the Muslim enclave of Srebrenica. After that, the shelling of Sarajevo got deadlier, provoking the NATO ultimatum. The Dutch have still not been allowed into Srebrenica and Tuzla airport remains closed.