OSTROZUB, Yugoslavia (AP) - Serb forces yesterday drew back from former Kosovo battlefronts, holding off the immediate threat of NATO airstrikes. But the 16-nation military alliance reserved the right to launch an attack anytime if the situation worsens.

To keep the pressure on Yugoslav President Slobodan Milosevic, the alliance decided to keep more than 400 NATO warplanes on alert but did not set any new deadlines for airstrikes after the previous one expired yesterday.

''We know that President Milosevic only moves when he is presented with the credible threat of force,'' NATO Secretary-General Javier Solana told a news conference after a 2 1/2 hour meeting of NATO ambassadors in Brussels, Belgium.

On Monday, more than 4,000 Serb policemen were reported to have pulled out of Kosovo, vacating many villages and dug-in positions.

''We have observed what I would consider significant reductions in the Serbian police presence in the field,'' said Shaun Byrnes, head of the American section of the Kosovo Diplomatic Observer Mission. The mission will be part of a group of 2,000 ''ground verifiers'' to make sure Milosevic is living up to his promises.

''Last night, we watched convoys of Serbian police leaving Kosovo and going north into Serbia proper,'' he said. ''We estimated that there were roughly 4,100. That's quite a few policemen.''

The Clinton administration reported ground observations indicated Milosevic was in ''substantial compliance,'' meaning the threat of allied military intervention has been averted.

Secretary of State Madeleine Albright said NATO would move quickly to use force against Yugoslavia if Milosevic sends troops and special police back to Kosovo. NATO allies were preparing to assemble a rapid-reaction force in neighboring Macedonia.

The removal of policemen encouraged a few among the 300,000 displaced people to take a chance on returning home. Avdullah Krasniqi, an ethnic Albanian from Ostrozub, loaded his farm wagon with wife, kids and household goods and headed home yesterday.

He decided to move because he believed his refuge would become too cold.

''We spent three months in the mountains,'' Krasniqi said as family members sat in a wagon on a huge pile of carpets, kitchen utensils and other household goods. ''It's very cold.''