Army Strikes Intensify Across Croatia : Yugoslavia: Zagreb residents race to shelters as air-raid sirens sound. Shells fall on Dubrovnik.

Air-raid sirens wailed in the Croatian capital Saturday for the first time in more than a week as the Yugoslav federal army intensified attacks across the breadth of Croatia.

Air force bombers strafed the city of Osijek, striking at the industrial heart of eastern Croatia, and federal troops in the hills above Dubrovnik unleashed a barrage of artillery shells on the historic Adriatic resort.

Hotels and vacation homes just north of Dubrovnik’s walled old town were set ablaze by army infantry units firing from less than a mile away, according to Croatian media. Federal warships blocking Dubrovnik’s sea lanes had blasted the surrounding districts for five hours overnight.

About 50,000 men, women and children have been trapped inside the medieval city for more than a month without electricity or telephone service. They are also reported to be short of food and drinking water.

Renewed fighting broke out within 35 miles of Zagreb early Saturday, with mortars tearing through the pockmarked baroque facades of central Karlovac and republic national guardsmen fighting the army in the woodlands around Sisak. A two-month battle between federal troops and 15,000 Croats holding out in the eastern city of Vukovar continued.

Zagreb Radio said republic forces had downed a MIG-21 fighter plane over the village of Saborsko, about 65 miles southwest of the Croatian capital. Air force flights that close to Zagreb probably triggered the air-raid warning, which sent residents scurrying to bomb shelters until the all-clear sounded an hour later.

At least nine people have been killed since the federal onslaught intensified Friday. Croatian Radio said a woman and four children died trying to escape the Dubrovnik siege by car, and four people were reported dead in Osijek.

There had been a relative lull last week in the fighting, which broke out after Slovenia and Croatia declared independence June 25 from the Yugoslav federation. Croatian authorities say the four months of civil war has cost 5,000 lives and forced nearly 400,000 people from their homes in the battle-torn areas.

Croats fear that attacks on their territory will escalate as a Tuesday deadline approaches for rival Serbia to accept a European Community plan to end the fighting or face economic sanctions.

The EC, meeting Tuesday in The Hague, is urging all six republics to respect each other’s sovereignty within the borders that defined them under the Yugoslav federation. Serbia, backed by the federal army, has made clear that it has no intention of complying with the EC proposal. It insists that ethnic Serbs living in other republics have the right to stay united within a single Yugoslavia.

Serbian President Slobodan Milosevic has vowed that his republic will not cave in to a foreign “ultimatum.” The federal defense minister, Gen. Veljko Kadijevic, has described the EC plan as a Western attempt to destroy the Balkan federation.

If Serbian republic and federal authorities have decided to risk economic isolation, which could shut off the fuel tap this winter, they would have little to lose with an all-out offensive to seize the last stretches of territory they want for a Greater Serbia.

About one-third of Croatia has been conquered by the Serbian-commanded army and occupied by Serbian militants opposed to independence for Croatia. The Serbs accuse Croatia’s leadership of plotting genocide against the republic’s 600,000 ethnic Serbs, and federal army leaders have justified their offensive as an attempt to prevent ethnic atrocities.

Croatian authorities charge that the Serbs are using the ethnic issue as a political cover while carrying out a land grab.