BUJANOVAC, Yugoslavia — Yugoslavia postponed a deadline Monday it had set for NATO to curb Kosovo Albanian militants on the Kosovo border — saying it wanted to give diplomacy a chance before launching a counterattack to drive out rebels who have killed four Serb policemen.

The new, democratic government of President Vojislav Kostunica had threatened to launch a counterattack on its side of the boundary unless NATO stopped rebel infiltration by 7 p.m. (11 a.m. MST) Monday.

But a Serbian deputy prime minister, Nebojsa Covic, said that the deadline was extended indefinitely after the NATO-led Kosovo Force "demanded that we not use the language of deadlines and ultimatums but that of diplomacy and agreements."

"So, everything is simply postponed," he said after visiting Serb police positions near the village of Lucane.

NATO-led peacekeepers said they had been aiding negotiations between the militants and the Serb police. U.S. Staff Sgt. Patrick McGuire, a NATO spokesman, said both sides had agreed to a cease-fire until Friday.

Covic said the NATO force in Kosovo "bears the full and exclusive responsibility" for making sure the ethnic Albanian militants withdraw from the area.

Kosovo is a province of Serbia, Yugoslavia's main republic, but it has been under international control since June 1999 under an agreement that ended NATO's 78-day bombing of Yugoslavia. The air campaign was launched to stop former Yugoslav President Slobodan Milosevic's crackdown on Kosovo Albanian separatists.

The crisis erupted again last week when ethnic Albanian militants of the "Liberation Army of Presevo, Medvedja and Bujanovac" attacked Serb positions in the Presevo Valley in order to unite the area with Kosovo.

Although the area has a substantial ethnic Albanian population, the valley was not considered part of Kosovo and therefore was not included in the agreement that sent NATO peacekeepers into the Serbian province. Serb police say they will use all available means, including heavy weapons, to regain territory lost to the militants, who killed four policemen Tuesday and seized police positions near the boundary between Kosovo and the rest of southern Serbia.

The government believes the attacks in southern Serbia were launched by ethnic Albanian extremists operating from Kosovo, which has been under NATO and U.N. control for nearly 18 months.

"The Serbian government will do everything to resolve the crisis by political and peaceful means," said Stevan Nikcevic, one of Serbia's three co-ministers of the interior. "At the same time, we have to protect the territorial integrity of the country so the terrorists don't jeopardize any lives of our citizens."

Despite the decision to extend the deadline, the government continued bringing in reinforcements in case diplomacy fails.

Heavily armed Serbian security forces, including their dreaded special anti-terrorist unit called SAJ, were seen moving near the three-mile demilitarized zone between Kosovo and southern Serbia.

On Sunday, Yugoslav army T55 battle tanks and armored personnel carriers could be seen maneuvering near the buffer zone.

Although the area has a substantial ethnic Albanian population, the valley was not considered part of Kosovo and therefore was not included in the agreement that sent NATO peacekeepers into the Serbian province.

The incidents cast doubt on NATO's ability to control Kosovo and also present a major crisis to Kostunica's government, which must defend the area without provoking the same international condemnation that accompanied Milosevic's crackdown.

In Vienna, Austria, on Monday, Kostunica denounced the United Nations and NATO for their stewardship of Kosovo. In an address to the Organization for Security and Cooperation in Europe, Kostunica claimed ethnic Albanian "terrorists" were trying to intimidate Serbs and Albanians alike.

He said it was "crystal clear" that NATO and the United Nations "failed to do their job properly" by curbing Kosovo Albanian militants.

Kostunica said he was cutting short his stay in Vienna because of the deteriorating situation in southern Serbia. He warned the crisis there "could set the whole region ablaze" and scuttle attempts to restore stability to the southern Balkans.

In Kosovo, Peter Deck of the U.N. refugee agency said that more than 2,000 people, mostly ethnic Albanians, have trickled out of the troubled region since the crisis started.