ZAGREB, Croatia -- NATO warned the Bosnian Serbs yesterday that if they did not accelerate the withdrawal of heavy weapons ringing Sarajevo in the next 24 hours, attacks by NATO warplanes and missile strikes on Serbian positions would resume.
United Nations officials said the Bosnian Serbs had withdrawn only a dozen artillery pieces and tanks from the heights overlooking the city, despite promises made Thursday to pull out some 200 heavy weapons in exchange for an end to NATO air attacks on Serbian positions.
By late last night, the United Nations reported that 43 heavy weapons had been moved past the 12.5-mile exclusion zone established by the United Nations. But it was unclear whether the Bosnian Serbs would comply with the U.S.-brokered agreement that has brought a 72-hour pause to two weeks of NATO attacks.
The lack of progress in the removal of the weapons came as the Serbs also blocked an aid convoy from entering the Bosnian capital, reneging on a promise to open one of two routes into the city.
"I hope the Bosnian Serbs realize the gravity of the situation," said Philip W. Arnold, the chief spokesman for the U.N. mission in the former Yugoslavia.
"The U.N.-NATO pause is designed to allow for the removal of a substantial number of heavy weapons by tomorrow night. Very little of this has been accomplished. The commitment of the Bosnian Serbs certainly appears not to have been met."
But U.S. Defense Secretary William J. Perry, speaking at a news conference in Ljubljana, the capital of Slovenia, was optimistic. "There are preliminary indications that the Bosnian Serbs are removing their heavy weapons" from the exclusion zone, he said. "We will continue to watch that very, very closely in the days ahead. But preliminary indications are positive."
The lack of progress dismayed U.N. officials who said that local ++ Bosnian Serb commanders may have decided, as they have in the past, to ignore steps agreed to by their own leaders and the international community to halt 3 1/2 years of fighting in the former Yugoslavia.
U.N. officials also speculated that the Bosnian Serbs might be testing the resolve of the Western allies to resume airstrikes in the face of heavy Russian opposition.
Under the current agreement, the Serbs must pull back about 200 of the total 350 to 400 weapons that U.S. intelligence experts estimate they have placed around Sarajevo.
Bosnian Serb forces, objecting to the use of a NATO Rapid Reaction Force sent to secure the Hadzici road into Sarajevo, also refused yesterday to allow a U.N. aid convoy to enter the city.
The U.N. High Commissioner for Refugees changed the route for one of the aid convoys after hearing that angry Serbian civilians would be waiting in the town of Hadzici, said Mans Nyberg, a U.N. spokesman.
He described the move as a tactic that has been used before by the Bosnian Serbs to harass aid convoys. He said that the convoy went through Kiseljak without incident. "We hope the Serb police will get their civilians under control," he said.
If NATO and the United Nations see progress by the Bosnian Serbs in carrying out the agreement by tonight, another 72-hour pause in the bombing would take effect. NATO warplanes carried out 3,500 sorties over two weeks against the Bosnian Serbs, the biggest combat mission in the 46-year history of the alliance.
In Slovenia, Mr. Perry said that the United States had suspended a plan to position Stealth fighter jets in Italy for use against the Bosnian Serbs.
"At this time we do not plan to send the F-117s over," Mr. Perry said. "We may, or may not, want to do that at some time in the future."
"It might be a couple of months, but if this process stays on track, I believe we will have a peace agreement this year," he said.
The U.S.-led peace effort would give the Muslim-led Bosnian government and its Croatian allies 51 percent of Bosnia, and the Serbs, who now control about two-thirds of the country, 49 percent of the territory.
If a peace settlement is reached, the Western allies would send two to three divisions -- more than 50,000 soldiers -- to ensure compliance with the plan. The United States is expected to provide up to 18,000 troops for the multinational force.