Allies Talk of Using Force to Aid Sarajevo : Balkans: Skepticism greets latest pledge by Serb militia to stop pounding civilian targets.

Even while watching a persistent United Nations general pursue his dogged quest for a cease-fire, the United States and its allies stressed Thursday the growing likelihood that a military thrust will be needed to save the starving and sick in battered Sarajevo.

In that besieged city, Canadian Maj. Gen. Lewis MacKenzie, commander of the U.N. peacekeeping force, announced that the Serbian militia had agreed “to unilaterally and unconditionally stop firing on non-military targets.”

MacKenzie said that U.N. observers will be allowed to monitor the Serbian artillery that has devastated Sarajevo, the capital of Bosnia-Herzegovina, and prevented relief planes from landing. Later, the Serbs promised, they would move these weapons “even further from the city.”

But the announcement, not surprisingly, was met with a good deal of skepticism. Cease-fires based on Serbian promises have been announced before, only to shatter within a day.

Supplying the food and medicine peacefully, said a senior State Department official, “remains our focus, but we understand that if this drags on, we may have to look at other things, including the use of military force.”

Russian Ambassador Vladimir P. Lukin made it clear that his country is willing to join a military operation if one is needed.

Lukin rejected a suggestion that the North Atlantic Treaty Organization step in to do the job by itself. “Why do you consider that Russia should play a passive role and NATO should intervene by itself?” he asked. “There are other organizations. . . . We should discuss it, all of us.”

Noting that the United Nations has stated for the last two weeks that it is waiting for 48 full hours of cease-fire before moving to occupy the Sarajevo airport, an American diplomat at the United Nations in New York said in frustration: “We start this 48-hour clock every three minutes.”

The frustrations were etched by Shashi Tharoor, a special assistant to Marrack Goulding, the U.N. undersecretary general in charge of peacekeeping operations. “You negotiate something with some party,” Tharoor said. “But then you have drunken soldiers down the road who say: ‘We don’t care who you negotiated with; hand over your trucks.’ ”

Bush Administration officials said it remains likely that some kind of military force will be required to protect supply flights or convoys into the besieged city.

Secretary of State James A. Baker III’s announcement Wednesday of a new set of diplomatic sanctions against Serbia was a last-ditch attempt to “turn up the heat on the Serbs” to get them to allow a peaceful resupply, one senior official said. But he acknowledged that “there may be no other way of getting humanitarian support in there . . . than by using force.”

U.S. officials want to make sure that if force is used, it is part of a clearly limited mission to protect humanitarian relief--not an open-ended commitment that could draw U.S. forces more deeply into the Yugoslav war.

The problem, one official said, is that protecting a humanitarian mission probably requires clearing the hills around Sarajevo of Serbian artillery units--an arduous job.

The official said there is little reluctance in the Administration to consider the use of airplanes or helicopters against the hostile positions but considerable resistance to putting troops on the ground.

Both civilian and military officials speak frequently of the ominous parallel to the U.S. military intervention in Lebanon in 1983 and 1984, when a Marine detachment secured the Beirut airport but found itself a target of artillery, snipers and terrorist bombs.

In Sarajevo, Gen. MacKenzie appeared to have more confidence in the present cease-fire agreement than in previous ones. “This is a significant commitment by the Serbs. . . ,” he told a news conference. “If it fails because of their action, it will be a very serious setback to Serbian credibility.”

While a public outcry mounted, especially in Europe, as a barrage of television images revealed the carnage in Sarajevo, U.N. officials insisted that the wisest course still is the patient search for an agreement to open the airport.