Serb Troops Pull Back From Strategic Peak Near Sarajevo : Balkans: One officer calls occupation of Mt. Igman ‘finished.’ U.S., meanwhile, repeats air strikes warning.

Serbian soldiers left Mt. Igman by entire busloads Wednesday, and U.N. officials said that after diplomatic dickering and bizarre deployments, the Serbs finally were making good on a week-old pledge to vacate the strategic peak dominating Sarajevo.

“They are moving in a southeast direction,” Cmdr. Barry Frewer of Canada, spokesman for U.N. peacekeepers in Bosnia, said with caution acquired through bitter experience. “That would seem to suggest that they are withdrawing from the area.”

Clad in olive-drab battle dress, at least 400 Serbian soldiers, some visibly furious at being ordered out, others just as obviously elated, came down from the roughly 4,900-foot-high peak to board buses in Trnovo, a Serbian-held town at the base.

They were reportedly bound for Banja Luka, a Serbian stronghold in northern Bosnia.

“It’s finished with Igman,” Lt. Drago Grubesic, a transport officer in the Serbs’ 1st Krajina Brigade, told the Associated Press. “But that doesn’t mean we won’t be back if necessary.”

Grubesic told the news service that about 2,500 soldiers had left Igman and nearby Mt. Bjelasnica, most of them Tuesday. Seven hundred more were supposed to come down later in the day, he said, but 1,500 will remain on the mountain until U.N. troops take control, probably within a week.

Igman’s significance goes far beyond the fact that the rocky, rugged chunk of real estate commands the southwestern approach to Bosnia’s capital, where more than 350,000 people have stubbornly held out from the Serbs during a punishing 16-month siege.

As long as Igman is in Serbian hands, Bosnia’s Muslim president, Alija Izetbegovic, has said he will not take part in negotiations in Geneva seeking a peaceful end to Bosnia’s war.

In addition, NATO has threatened to unleash its air power against the Serbs unless they quit the two mountains they seized last week and lift their blockade of Sarajevo as well.

In Washington, the U.S. government repeated those warnings Wednesday.

“I don’t want to fix a precise time deadline, but it’s a kind of situation where that’s a very significant event to us as to whether they give up those mountaintops,” Secretary of State Warren Christopher said in a television interview.

Christopher said NATO’s plans for an air attack on the Serbs could be implemented rapidly. NATO, in approving the plans Monday, gave U.N. Secretary General Boutros Boutros-Ghali the final decision for such use of force.

Mike McCurry, the State Department spokesman, said that if earlier reports of the Serbs’ bringing fresh troops and weapons onto the mountains were accurate, then “all the criteria set forth by NATO as preconditions for using more aggressive military measures will have been met.”

Bosnian political leader Radovan Karadzic promised last Thursday to evacuate the mountains in order to lure Izetbegovic back to the talks and the plan under consideration there to chop Bosnia into three ethnically based ministates.

But then the Serbs and Izetbegovic squabbled over the fine print specifying who should replace the Serbs--Bosnian government soldiers or U.N. troops.

On Wednesday, international negotiators Lord Owen and Thorvald Stoltenberg told Karadzic that the withdrawal had to be complete by this morning, reports from Geneva said. The implication seemed clear: Do it or risk NATO air raids.

Karadzic claimed to reporters in the Swiss city that the withdrawal from Igman was already more than half complete. Asked how long it would last, he answered: “It all depends on the U.N. and their capability to take control of the mountain. We are willing to withdraw in two hours, but the U.N. does not have enough troops.”

On Tuesday, the British news agency Reuters quoted French peacekeeping troops as saying that the Serbs had retaken and reinforced positions on Igman that they had abandoned the previous day. Karadzic dismissed that as a “complete lie.”

“One of our units, tired and wet because of the rain, was delayed, but they are progressively leaving the mountains,” he said.

The U.N. commander in Bosnia, Gen. Francis Briquemont, said in Sarajevo that the news agency’s dispatch had been “totally in contradiction” with reports from a French battalion sent to the peak to check on the Serbs’ actions.

He acknowledged that U.N. observers had observed troop movements that he could not explain, but he said he thought it would take only another day or two for the Serbs to pull out entirely.

Explaining the maneuvers that had appeared to some like a reoccupation of the mountain, the Bosnian Serb army command said it was simply replacing “exhausted” units until U.N. Protection Force troops took over the territory as a buffer between Serbian and Muslim-led forces.

In Sarajevo, commanders for the three belligerents in Bosnia’s civil war--Serbs, Croats and Muslims--ended 18 hours of talks by signing a wide-ranging plan for curtailing military operations in the former Yugoslav republic.

The accord contains a major catch: It will go into effect only if a peace deal is concluded in the stalled Geneva talks. And that seems impossible if Izetbegovic maintains his opposition to the proposal to carve up Bosnia.