5 Truce Monitors Killed as Yugoslavs Down EC Copter : Civil war: Missile fells the craft over Croatia. Foreign efforts to halt the fighting are thrown into doubt.

A fighter jet of the Yugoslav army blasted a European Community helicopter out of the sky over Croatia on Tuesday, killing all five cease-fire monitors on board and further jeopardizing foreign efforts to end Yugoslavia’s civil war.

The attack, for which the Serbian-led federal Defense Ministry in Belgrade expressed “deep regret,” shattered the relative peace that had reigned in Croatia since a 15th truce took effect Friday.

It also enhanced fears that renegade factions of the federal military might try to provoke new conflicts in Yugoslavia ahead of an EC plan to recognize Croatia and Slovenia as independent countries as of next Wednesday.

Four Italian soldiers and a Frenchman died in the fiery crash near the border with Hungary after the federal jet fired air-to-air missiles at their EC craft. It exploded, spewing wreckage over the Croatian village of Madjarevo, 30 miles northeast of Zagreb. They were the first EC monitors killed from an observer force of nearly 200 in more than six months of war between breakaway Croatia and the federal army fighting on behalf of Serbia.

EC diplomats in Zagreb said the downed craft and another EC helicopter carrying four monitors, including a Belgian diplomat, were en route from Belgrade to Zagreb when the attack occurred. The second helicopter crash-landed in Madjarevo after escaping the missile firings. All four passengers survived, but the chopper was damaged.

“I saw the trail of the rocket, then there was a great fire, an explosion,” said Davor Pejnovic, 27, a reporter for Radio Novi Marof who witnessed the attack.

EC officials in Brussels said they were sending a team to investigate the incident, and future efforts to broker peace between warring Serbs and Croats would depend on the circumstances of the attack.

The state-run news agency Tanjug in Belgrade, which is both the federal and Serbian capital, distributed a Defense Ministry statement confirming that a federal jet shot down the EC helicopter. It said the military hierarchy expressed “deep regret,” that an investigation has been ordered and that those responsible for the “unwanted and tragic event” will be punished.

It was unclear why Yugoslav warplanes were flying so deep into Croatian territory, and it would be difficult for federal pilots to claim the downing was a mistake. Croatia has no combat aircraft, the air force was supposed to be observing a cease-fire and the copters used by the monitors are painted white and bear the EC’s distinctive ring of gold stars on a navy-blue field.

The deadly incident occurred at a fragile juncture in the Yugoslav war, just as foreign mediators were beginning to put some faith in the latest cease-fire aimed at stopping the fighting that broke out after Slovenia and Croatia declared their independence last June.

A secure truce is a precondition for deployment of a U.N. peacekeeping force, which both Serbian and Croatian leaders claim to want.

Some speculated that military hard-liners opposed to U.N. intervention were trying to scuttle the cease-fire, since the army would effectively be made redundant if foreign troops are called in to prevent further ethnic conflicts.

The attack may also have been aimed at further discrediting Yugoslav Defense Minister Veljko Kadijevic, who has already been forced to apologize for a brutal and apparently renegade assault on the Adriatic resort of Dubrovnik in December.

U.N. special envoy Cyrus R. Vance had negotiated the latest truce and made clear that its success was essential to permit intervention of up to 10,000 U.N. peacekeeping troops to separate the combatants.

On Monday, U.N. Secretary General Boutros Boutros-Ghali said he would send an advance team of 50 military observers to Yugoslavia, but again said a peacekeeping deployment was contingent on a lasting cease-fire.

Both Serbia and Croatia have appealed for U.N. intervention in the war, which already has taken 10,000 lives. The federal army has formally agreed to abide by conditions for foreign intervention.

But Serbian extremists have balked at Belgrade’s agreement to withdraw federal soldiers from Croatia at the same time the United Nations moves in. Milan Babic, leader of the Serbian-inhabited Krajina region of southwestern Croatia, has vowed to fight the dispatch of foreign troops to the territory he controls.

European diplomats reacted cautiously to Tuesday’s tragedy, although some privately expressed concern that it would hamper the peacekeeping efforts.

Boutros-Ghali said he was grieved by the deaths of the monitors, but he added in a statement issued at the United Nations’ New York headquarters that “this tragic incident vividly demonstrated the need to redouble the efforts under way to bring peace back to Yugoslavia.”

The spokesman for the EC monitoring mission in Croatia, Portugal’s Joao da Silva, said an investigation has been launched and an EC delegation has been immediately dispatched to the scene of the crash, which occurred about 2 p.m. “Pending outcome of the investigation into the incident, it would be inappropriate to say anything at this stage,” Da Silva told reporters in Zagreb.

At a news conference in Lisbon, the capital of Portugal, which now holds the rotating EC presidency, British Foreign Secretary Douglas Hurd paid tribute to the slain monitors as courageous men performing a dangerous job in the name of peace.

But in Rome, Deputy Foreign Minister Claudio Vitalone accused the attackers of “a premeditated act of sabotage against the latest efforts in the search for peace.”

EC and U.N. diplomats have closely cooperated in efforts to restore peace in Yugoslavia, but continued involvement by Western European countries, in particular Germany, has angered Serbia and may have played a role in instigating the attack on the helicopter.

Serbia vehemently opposes the EC’s stated intention to extend diplomatic recognition to Slovenia and Croatia next week.

Times staff writer John J. Goldman contributed to this report. Special correspondent Kirka reported from Zagreb and staff writer Williams from Vienna.