Croatia and Serbian-Led Army Agree to New Cease-Fire : Yugoslavia: They yield to threat of a pullout by European monitors. The plan contains elements previously rejected by one side or the other.

Under the threat of a pullout by European Community monitors, Croatia and the Serbian-led federal army agreed to a new cease-fire plan Tuesday only seconds before a midnight deadline.

While the eighth truce to be proclaimed in Yugoslavia’s deadly three-month civil war is likely to put off fighting for a day or two, the six-point plan contains many of the same conditions that one side or the other has found impossible to accept under previous cease-fires.

An atmosphere of an agreement forged under pressure stirred fears that the combatants would fail to uphold the truce, or that they had signed only to put off what now appears to be an inevitable cataclysm between the bitter rivals.

Army officials had threatened to unleash a punishing attack on Croatian cities if Croatian forces did not meet a midnight-Tuesday deadline for lifting a blockade of federal bases.

After Monday’s precision attack on Croatia’s presidential palace, which narrowly missed republic President Franjo Tudjman and the federal prime minister and head of state, Croats have begun to fear that their capital and vital seaports could suffer heavy bombing.

“It’s pay time,” a Belgrade Serb said, explaining the escalating conflict. “Now the Croats are being made to pay for what they did to Serbs during World War II.”

Serbian militants are opposed to an independent Croatia because of the atrocities committed by the last free Croatian state--a Nazi puppet regime during World War II. They have seized nearly half of Croatia’s territory in a guerrilla war that has rekindled old ethnic hatreds.

Croatia’s first response to the army ultimatum Tuesday was to declare its intent to follow through on plans to secede from Yugoslavia.

It was the June 25 declaration by Croatia and neighboring Slovenia that they were pulling out of the troubled federation that triggered fighting that has killed at least 1,000 and brought economic devastation to most of the republics.

Seeking to halt the advance of the rebels, who are backed by the powerful federal army, Croatian national guardsmen have surrounded several federal garrisons in the republic in recent weeks, cutting off electricity, fuel and food supplies.

The blockades have spurred mass desertions and inflicted casualties among the federal forces, prompting the army to launch a major offensive against Croatia over the past few days.

Meanwhile at the United Nations, Secretary General Javier Perez de Cuellar appointed former Secretary of State Cyrus R. Vance as his personal envoy to help arrange a peaceful settlement, news agencies reported.

The appointment of Vance, experienced in Vietnam peace negotiations and the Cypriot and Korean conflicts, came in response to an appeal by European Community foreign ministers meeting Sunday in The Hague.

Vance is to meet Thursday at the United Nations with Perez de Cuellar and Lord Carrington, chairman of the Hague Peace Conference on Yugoslavia. He then is to visit Yugoslavia, meet parties to the conflict and report back.

Under the accord hammered out after 10 hours of tense negotiation in a Zagreb hotel Tuesday, all shooting would be expected to cease as long as peace negotiations are in session in The Hague.

By Saturday, Croatian forces would have to lift their siege of the Boronaj federal barracks near Zagreb, and in return the federal Defense Ministry would be required to end its naval blockade of Croatian ports.

The agreement further requires free movement of food and medical supplies to besieged areas of eastern Croatia and continued monitoring of compliance by European diplomats.

“This is the first concrete perspective we’ve had for reaching a peaceful solution,” Dirk Jan van Houten of the Netherlands, head of the European Community’s monitoring mission, told journalists just before midnight.

But Croatia’s increasingly belligerent forces are expected to resist the order to free the tanks and troops bottled up at federal bases. They fear that federal firepower could be turned against them and used to seize more territory.

An end to the Croatian siege of bases had been agreed to under a peace plan approved Friday by the army, Croatia and Serbia. But the combatants immediately imposed new conditions on the agreement after it was announced, raising the likelihood that similar resistance will be encountered in implementing the latest accord.

The European diplomats have been frustrated by the fast failure of seven previous cease-fires. They warned the combatants in the civil war that they would leave if a solid truce was not reached.

Although the EC monitors’ presence in Yugoslavia seems to have had little positive influence, their withdrawal could have cataclysmic consequences.

With no foreign presence to deter aggression or observe violations, both sides might use the opportunity for new attacks.