Serbs Defy NATO, Advance on Gorazde : Bosnia: U.N. refuses to approve air strikes despite daylong assault. Then, as deadline for pullback nears, rebels show signs of complying with alliance’s ultimatum.

Bosnian Serb infantry advanced defiantly on the strangled eastern city of Gorazde on Saturday in violation of a NATO ultimatum to end their slaughter of civilians or face punishing air strikes.

U.N. officials confirmed that the rebels were still attacking more than 24 hours after the Western alliance vowed to bomb them unless they halted their fire immediately. But the civilian chief of the U.N. peacekeeping mission here vetoed a NATO call for air power.

Early today, the United Nations said that no immediate NATO air strikes had been ordered because the Serbs were apparently complying with the ultimatum to withdraw from the enclave. But U.N. officials could not confirm full compliance.

Artillery shelling of the bedraggled enclave packed with refugees from the 3-week-old offensive was reported to have died down around a 2:01 a.m. deadline for withdrawal, and a regional official in Gorazde reported by ham radio that there were no more tanks visible in the city center. The rebels also allowed a U.N. humanitarian aid convoy to enter Gorazde late Saturday night.

For most of the day Saturday, the nationalist rebels appeared bent on pushing the 16-nation North Atlantic Treaty Organization to deliver on its threat of air attacks--a move that would enmesh Western governments in the deadly Balkan conflict.

Bosnian Serb soldiers, backed by heavy artillery, moved deeper into the proclaimed withdrawal zone from the north just before noon in what one U.N. observer in the enclave described as “blatantly increased aggressive activities in the face of the NATO ultimatum.”

They concentrated their firepower on an important weapons complex, seizing at least one-third of the vast plant and looting it of munitions.

In the 24 hours after NATO ordered the Serbs to retreat, at least 21 people were killed in Gorazde by tank, artillery and machine-gun fire. Even after Serbian forces fired tank rounds into the maternity ward of Gorazde’s already demolished hospital, killing at least three patients, Yasushi Akashi, the civilian chief of the peacekeeping mission, refrained from authorizing air strikes.

The NATO ultimatum issued Friday demanded that the Serbs stop attacks on Gorazde immediately; pull back at least 1.9 miles from the city center by 2:01 a.m. local time today (5:01 p.m. Saturday PDT), and move heavy weapons outside a 12-mile “exclusion zone” by 2:01 a.m. Wednesday (5:01 p.m. PDT Tuesday). NATO said that failure to meet these requirements could bring an air attack.

U.N. Secretary General Boutros Boutros-Ghali issued a statement when the 2:01 a.m. deadline expired, saying he would “immediately authorize the use of air strikes to protect the safe area of Gorazde” if the Serbs failed to abide by a pullout agreement ratified in Belgrade earlier in the day.

But there was no indication from the U.N. command center here that British Lt. Gen. Michael Rose, the commander for Bosnia-based troops, had called for punitive action against recalcitrant Serbian gunmen still in the exclusion zone.

“There is finally some evidence that the threat of NATO air strikes is having an effect and the Bosnian Serbs are beginning to comply” but need “more time to pull back,” a senior Clinton Administration official said a few hours before the expiration of the deadline.

The United Nations’ insistence that the Serbs be given more time to comply meant it was unlikely that any air strike would be launched before daybreak today at the earliest, another Administration official said.

NATO officials had pushed for air strikes against Serbian targets earlier Saturday as it became clear that the rebels were inflicting further casualties and destruction on the city designated a U.N. safe area.

They said Akashi, who was in the Yugoslav capital of Belgrade earlier in the day, had refused the call for air strikes, instead holding out for eleventh-hour signs the rebels have been cowed into compliance. Any attack must be approved by both NATO and U.N. mission officials.

Bosnian Prime Minister Haris Silajdzic angrily denounced Akashi for vetoing intervention that might have saved lives in Gorazde, where 700 people have been killed in the Serbian offensive and 2,000 have been wounded.

“We do not understand why the U.N. on the ground here would not give this authorization since they know very well what happened today in Gorazde,” Silajdzic said, saying he held Akashi and Rose personally responsible for the latest civilian deaths.

Earlier in the day, the American, British and French governments ordered the evacuation of all nonessential personnel and family members from their embassies in Belgrade, in fear that any air strikes would provoke retaliation against Westerners.

Bosnian Serb gunmen took nearly 200 U.N. peacekeepers hostage and hampered the work of thousands last week after two token NATO air strikes against Serbian artillery firing on Gorazde.

The head of Gorazde’s regional governing council, Esad Ohranovic, said the city hospital and a refugee center were targeted again Saturday.

“The hospital is almost in ruins. We have no medicine, no blood, no food,” he said via ham radio.

He confirmed reports by U.N. officials that Bosnian Serb gunmen were concentrating their attacks on the Pobjeda munitions factory within the 1.9-mile radius of the city center NATO ordered the rebels to withdraw from.

After raking the arms complex with artillery and machine-gun fire all day, the rebels moved about 100 trucks into the factory grounds and began looting machinery and munitions stores, one U.N. source confirmed.

Ohranovic said three factory workers were injured when a shell blasted through the building they were working in only an hour before the withdrawal deadline.

U.N. mission spokesman Cmdr. Eric Chaperon described the situation in Gorazde as “confused” and confirmed that the refugee-packed city center was still under heavy artillery fire late Saturday, in open violation of NATO’s edict for an immediate halt to the Serbian onslaught.

Despite the continuing hostilities, a convoy of about 200 U.N. troops from Ukraine, France and Norway were deployed to Gorazde late in the evening, circling the ravaged city center before taking up positions near the building where a handful of U.N. military observers have been hunkered down against the siege.

A smaller convoy of peacekeepers had tried to reach Gorazde two days earlier but was blocked about 18 miles to the north by a crowd of Bosnian Serb women and children.

Hundreds of badly wounded civilians were awaiting medical evacuation from Gorazde, but a reliable cease-fire and some U.N. troops on the ground were needed before helicopters could be flown into the desperate pocket.