U.N. Controls Sarajevo Airport; 1st Flight Lands : Balkans: More peacekeepers are on the way to secure the lifeline for 300,000 starving civilians.

United Nations peacekeeping troops hoisted their powder-blue flag over the Sarajevo airport Monday after Serbian guerrillas withdrew their heavy guns. Within hours, a French aid plane broke the three-month siege of the city by touching down with a cargo of food and medicine.

In New York, the U.N. Security Council acted on a call to “seize the opportunity” and quickly move in more peacekeepers to secure a lifeline to the starving city.

U.N. Secretary General Boutros Boutros-Ghali had given the Serbs 48 hours to hand over control of the airport to U.N. forces or face the prospect of foreign military intervention to loosen the stranglehold preventing supplies from reaching 300,000 civilian holdouts.

The ultimatum expired early Monday with many of the Serbian tanks and artillery pieces still surrounding the airport. But Boutros-Ghali and U.N. commanders here treated the slow but steady departure of Serbian hardware as a good-faith effort.

By nightfall, the last Serbian armor had withdrawn from the airport, and U.N. officials said a full pullout from the surrounding suburbs appeared to be under way.

The French aid flight, which had been waiting at the Adriatic port of Split less than 100 miles away, arrived three hours later with 6.5 tons of food and medicine.

Maj. Gen. Lewis MacKenzie, commander of the token U.N. force still in shattered Sarajevo, said that Serbian gunmen appeared to be complying with an agreement reached three weeks ago to place all heavy weapons under U.N. supervision beyond range of the airport.

At a session called Monday to weigh possible action to break the Serbian blockade of Sarajevo, the Security Council ordered a Canadian battalion of U.N. “blue helmets” currently in Croatia to divert to the capital of Bosnia-Herzegovina to protect the airport.

The unanimous vote came after Boutros-Ghali urged the council in a cable to “seize the opportunity” provided by the Serbian withdrawal, even though fighting around the airport continued. Shooting also was reported in the city center, about 10 miles to the east.

U.N. officials said they were hopeful that the dispatch of additional troops would assure both sides that major Western powers are serious about forcing an end to the fighting and are prepared to intervene more forcefully if a cease-fire does not succeed.

Although the Security Council resolution did not directly threaten the use of military force, it warned obliquely that if fighting continues to cut off food, medicine and other crucial supplies, the council “does not exclude other measures” to open a lifeline to the city.

Boutros-Ghali urged that countries seeking to donate supplies to Sarajevo wait until the airport has been fully secured and all tanks and artillery pieces have been surrendered to U.N. forces. He will reassess the situation Wednesday.

“At that point it will be for Gen. MacKenzie to see how well things are functioning, whether we can offer a security umbrella,” said Shannon Boyd, the U.N. spokeswoman in Belgrade. “But at least a minimum of structure has to be in place before (the U.N. peacekeepers) can authorize flights.”

Sarajevo’s Butmir Airport is vital for supplying the Bosnian capital because of the unusual geography of the city, which stretches for miles in a narrow valley along the Miljacka River, rarely extending more than a few blocks from the waterway.

Serbian forces control both the eastern and western road approaches, as well as the mountain ridges on both sides of the Miljacka.

If Serbs complete the removal of heavy weapons from the airport area at the city’s western end, they will probably be forced to retreat from the nearby suburb of Ilidza for lack of artillery cover. That could mean the opening of a major land route into the city, as Croatian and Muslim troops are reported to be advancing toward Sarajevo from western suburbs a few miles away.

Because the airport is key to supplying the city, its reopening would be a major setback for the Serbian cause of starving out the population and adding that capital, once home to 600,000, to the other land under Serbian control.

The relocation of the Canadian battalion from its base in Daruvar, Croatia, was expected to take a minimum of 48 hours from its midnight Monday departure, as the 1,000 troops and their equipment can move no faster than about 25 miles per hour, Boyd said.

Sarajevo has been cut off from air, rail and road traffic by Serbs rebelling against the proclaimed independence of Bosnia-Herzegovina. Seeking to remain aligned with Serbia in the shrinking Yugoslav federation, they reacted violently to a Feb. 29 referendum endorsing secession. Many Bosnian Serbs boycotted the vote that was assured of passage because of support from the republic’s Slavic Muslims and Croats, who together make up more than 60% of the 4.4 million population.

Serbs have seized two-thirds of the republic’s territory and driven out more than a million non-Serbs. The uprising has killed more than 7,400 and left 35,000 missing, many presumed dead.

The United Nations imposed harsh sanctions a month ago against Serbia and Montenegro, the last two republics still allied within Yugoslavia. The oil and trade embargo has had little visible effect on the mostly self-sufficient Serbs, but there is growing popular discomfort with Serbia’s international isolation and the seemingly endless ethnic conflicts that Serbian President Slobodan Milosevic is accused of instigating.

In Washington, the State Department signaled Monday that the United States was prepared to approve the use of force by the United Nations--possibly even including some U.S. troops--to get humanitarian aid to Sarajevo if the city’s airport remained under fire.

Margaret Tutwiler, the department’s spokeswoman, refused for the first time to rule out the possibility that American ground forces would be used in Bosnia-Herzegovina.

Asked before the Security Council voted Monday whether a cease-fire were still a necessary condition for sending aid, Tutwiler said that “if the United Nations votes on a resolution to take all necessary measures (to get aid to Sarajevo) . . . , it would be an action that we would support.”

Times staff writers Art Pine at the United Nations, Robert C. Toth in Washington and Douglas Jehl in New York contributed to this article.