U.N. Reopens Road to Sarajevo : Bosnia: Aid convoy into capital is greeted by cheering residents. NATO warplanes crisscross skies as the deadline nears for lifting of Serbian siege of the city.

As a tense diplomatic standoff persisted over the Bosnian Serb siege of Sarajevo, the United Nations tried to loosen the stranglehold Sunday by reopening a key mountain road into the Bosnian capital.

A convoy of a dozen trucks, carrying food and firewood, was greeted by waving and cheering residents as it entered the city near Sarajevo’s airport after descending along a muddy route over nearby Mt. Igman.

It was the first time since March that a U.N.-protected road into the city was opened to private traffic, although the route has been used almost continuously by military vehicles, journalists and diplomats.

The road is one of the most precarious into Sarajevo but had grown in popularity since the city’s airport was closed by the rebel Serbs in April. Three U.S. diplomats were killed last month when a crumbling section collapsed beneath their armored vehicle.

The trucks arrived Sunday as thundering NATO warplanes crisscrossed the skies over Bosnia-Herzegovina. But the aircraft did not renew their bombing campaign in the last-ditch hope that the recalcitrant Bosnian Serbs would submit to alliance demands.

U.N. officials, who last week ordered a punishing bombardment of rebel Serb targets, said the suspended aerial assault will start again if Bosnian Serb army commander Gen. Ratko Mladic does not end the siege of Sarajevo by tonight.

There were conflicting statements from the United Nations and the North Atlantic Treaty Organization about the exact timing of the deadline. But the top NATO commander in Southern Europe left no doubt Sunday that it was fast approaching.

“They have very little time in which to make a decision to start showing progress in this,” U.S. Adm. Leighton W. Smith said on NBC-TV’s “Meet the Press.”

“And it’s up to them as to whether bombing starts, if it should start again.”

The delivery trucks, under U.N. escort, met no resistance Sunday from the Bosnian Serbs, who have maintained a tight grip on the capital for 3 1/2 years and have frequently shelled the route to keep it out of commission.

U.N. officials said they notified the Serbs that the route would be reopened Sunday afternoon but did not request the customary approvals sought in the past.

Bosnian government officials and some analysts said the decision to reopen the so-called blue route reflected a new, bolder attitude on the part of the United Nations since the air strikes last week.

In the past, the United Nations was always careful not to offend the Bosnian Serbs; in the case of the newly reopened road, U.N. peacekeepers acted first and asked later. It is “part of a plan to open Sarajevo completely,” one Bosnian official said.

The United Nations said its rapid-reaction force was on hand to defend the convoy in the event of an attack, but television in Belgrade, the capital of Serbia and the rump Yugoslavia, reported Sunday that the Bosnian Serbs welcomed the U.N. effort to secure the road.

The Belgrade report, quoting Bosnian Serb official Momcilo Krajisnik, said U.N. troops were necessary to ensure that the Muslim-led Bosnian government did not try to sabotage the supply route--and blame the Bosnian Serbs.

“Opening the road is going to take away the trump card from the Muslims’ hands,” Krajisnik was quoted saying. “They can’t now use the closed roads as an excuse for not entering the peace process.”

Fragile peace efforts continued Sunday night with the arrival in Belgrade of Assistant Secretary of State Richard Holbrooke, the chief U.S. negotiator in the Balkans. Holbrooke and European Union envoy Carl Bildt had dinner with Serbian President Slobodan Milosevic to prepare for a high-level meeting of Serbian, Croatian and Bosnian leaders this week in Geneva, the most senior meeting among the warring sides in two years.

Holbrooke returned to Belgrade after a two-day whirlwind trip to Bonn, Brussels and Geneva, where he conferred with European leaders, NATO officials and representatives of the Organization of the Islamic Conference.

But even with the apparent speed of the peace effort, Mladic’s refusal to lift the siege on Sarajevo hangs like a black cloud over each incremental diplomatic achievement and underscores how delicate the peace process remains.

On Sunday, the Bosnian government threatened to pull out of the Geneva meeting because of the prolonged break in NATO’s air campaign against the Bosnian Serbs despite Mladic’s intransigence.

“They have let their finger slip off the trigger,” Bosnian Foreign Minister Muhamed Sacirbey charged. “The credibility that has been bought by the international community . . . is being lost by every hour of the pause.”

Holbrooke, in private and public statements, sought to reassure the Bosnian government that NATO had not lost its resolve to punish the Bosnian Serbs if Mladic refuses to back down. He also sought to ensure that Mladic’s defiance does not discredit Milosevic, the general’s patron in Belgrade, who is crucial to any negotiated settlement of the war.

“This bombing package, it has only just begun,” Holbrooke said in Geneva. “It is a big one. I know what it is. It will not be pleasant, but it will be unavoidable” if Mladic does not comply.

“President Milosevic has stated to us that he wants this to stop,” Holbrooke continued. “Let us assume that is correct--then the problem is Mladic.”

In a letter to Mladic on Sunday, the United Nations and NATO made several unconditional demands that would loosen the Bosnian Serb stranglehold on Sarajevo. They called for the withdrawal of heavy weapons from around the city; the opening of Sarajevo’s airport; an end to attacks on the city and other U.N.-designated “safe areas,” and the freedom of movement of U.N. and relief personnel.

Mladic set conditions on complying with the NATO demands in a weekend meeting with Gen. Bernard Janvier, the U.N. commander in the Balkans. NATO rejected them but opted to hold off on more air strikes.

It emerged Sunday in Brussels that NATO ambassadors were so outraged by Mladic’s evasive, noncommittal response that several alliance members had pressed for an immediate resumption of the air campaign. The generals’ meeting, scheduled for 90 minutes, lasted nine hours, and the decision was made to give the Bosnian Serb commander more time.

Under the rules of engagement, NATO’s Smith and his U.N. counterpart, Janvier, must jointly decide whether the Serbs have begun to comply with the demands and, if not, to resume air strikes.

NATO officials Sunday said more than 1,000 sorties have been flown over Bosnia in the five days since the first sustained combat use of alliance air power began Wednesday morning. While attacks were halted Friday, reconnaissance missions have continued in the area, as have search-and-rescue efforts for two French pilots, lost when their Mirage jet was hit by a Bosnian Serb surface-to-air missile Wednesday afternoon.

In other developments Sunday:

* The saga of five European Union monitors, thought to have been killed last week in the NATO bombardment, ended happily as the officials arrived safely in Belgrade and were en route to their homes in Spain, the Netherlands and Ireland.

On Saturday, Serbian television had reported that the men would not be released until NATO called off its military action. The monitors, all of them in good condition, told reporters that the Bosnian Serbs said they had been held for their own safety.

* Rebel Serbs in the Eastern Slavonia region of Croatia, the last piece of disputed territory in that country, have reportedly agreed to allow U.N. personnel to monitor the border between Eastern Slavonia and Serbia.

The Croatian government, which has threatened to retake the disputed region by force, has complained that the rebel Serbs have been armed and supplied by Serbia.

* In Moscow, German Chancellor Helmut Kohl said that Germany and Russia remain far apart on the use of NATO air strikes in the Balkans but agree that time is running out on efforts to bring peace to the region.

Times staff writer Tyler Marshall in Brussels contributed to this story.