With talks on the NATO-Belgrade peace deal at an impasse, U.S. B-52s bombed Yugoslav army positions in Kosovo just opposite this border town today as the alliance stepped up its air campaign against Serb-led government forces trying to blunt an offensive by separatist Kosovo Albanian guerrillas.

Thunderous explosions rumbled across the mountainous frontier terrain, and plumes of thick black smoke billowed over the horizon this afternoon shortly after two B-52s dropped their deadly loads in an area south of Mount Pastrik, the 6,523-foot peak where government security forces and the rebel Kosovo Liberation Army have been locked in a dogged artillery duel since May 26.

Ten minutes later, two more B-52s -- or perhaps the same two -- arched back over the area from the opposite direction and loosed another bomb load, apparently striking near Planeja, a village about three miles east of Mount Pastrik, which marks the border between Yugoslav and Albanian territories.

Rebel and Yugoslav units also clashed along the border about 30 miles north of here, between the Albanian town of Bajram Curri and the Kosovo town of Junik, an established rebel supply corridor into Kosovo -- a province of Serbia, Yugoslavia's dominant republic.

The NATO air attack came as allied officials promised an intensified bombing campaign to force the Belgrade government to abide by the peace plan it accepted last week. Discussions between military officials on the details of a withdrawal of Yugoslav forces from Kosovo broke down on Sunday, resumed today, then were recessed until Tuesday.

This afternoon's B-52 raids near Morina were the most visible part of what was nearly a daylong NATO overhead presence, always audible but often too high to be seen.

The brunt of the air attacks over the past 24 hours were concentrated on Yugoslav troops, Serbian special police and paramilitary units, armored vehicles and artillery positions in western Kosovo, where the guerrillas have launched an offensive from bases along the Albanian border, NATO officials told reporters in Brussels. The rebels' push has lured government forces out of entrenched positions, making them more vulnerable to NATO airstrikes.

On the 76th day of the bombing campaign, NATO warplanes flew nearly 500 sorties, hitting military targets across Kosovo that included a command post, 20 artillery pieces, 17 armored personnel carriers, 4 tanks and 15 other vehicles, the officials said.

Explosions were heard in Belgrade, and local radio said the huge oil refinery at nearby Pancevo was bombed. Two missiles slammed into houses in Belgrade's suburbs without exploding, the official Yugoslav news agency Tanjug said. State media also reported fatal strikes in Novi Sad and a village near Boljevac, about 110 miles southeast of the capital. But maps released by NATO showed that the majority of the attacks were carried out along the Albanian frontier.

A day earlier, an attack by a pair of B-52s and another pair of U.S. B-1B high-altitude bombers dropped a total of 86 unguided gravity bombs -- Mark 82s -- around the Mount Pastrik region, destroying major elements of two battalions of government forces, Pentagon officials said. They said the raid may turn out to be the heaviest single air attack of the 11-week-old conflict.

A spokesman for the Kosovo rebels said they will continue their offensive in the Morina border area until NATO suspends its military operations, but he warned that a rebel cease-fire would be contingent on a peaceful withdrawal of Yugoslav and Serbian forces from Kosovo.

"If we see them harming civilians or property when they withdraw, then we will attack them," said Ilir Rama, a spokesman for the rebels in Tirana, the Albanian capital. "If they leave peacefully, they can go."

Rama said front-line rebel units continued to exploit NATO bombing and had made limited advances in the Mount Pastrik area in the past 48 hours. He said the guerrillas' tactical goal remains seizure of sections of the main highway in western Kosovo, between the towns of Prizren and Pec, and ultimately the capture of Prizren.

Rama said the NATO bombing was causing heavy losses among government forces in Kosovo and that rebel infiltrators had witnessed infighting between Yugoslav and Serbian forces. "It is a dirty war on the other side," Rama said. "Some of the Yugoslav soldiers are running, but there are [Serbian militiamen] and special police behind their lines, and they are forcing them back to the front or killing them."

It was on the southern slope of Mount Pastrik, just north of Morina, that the rebels launched a major offensive two weeks ago, apparently hoping to establish a supply corridor for their forces deeper in Kosovo. The campaign has continued relentlessly despite heavy rebel losses and has spread in recent days to the northern side of the mountain, military observers said.

Casualty figures were not available for either side, but border observers with the 55-nation Organization for Security and Cooperation in Europe said that the rebels were ground down in the opening days of the offensive by land mines that the Yugoslav army has laced across the Albania-Kosovo frontier.

In recent days, however, the majority of casualties has been caused by artillery barrages fired high onto the mountain by Yugoslav forces east of Vrbnica, a village just a mile or so inside Yugoslavia. Last Thursday, more than 30 rebels wounded in the fighting were said to have been brought to a Norwegian field hospital near the Albanian border town of Krume.

Analysts have been perplexed by the rebels' choice of ground for their offensive, saying it is exposed and surrounded by heavily fortified Yugoslav positions. But despite the continuous punishment they are taking, observers said, the rebels have pushed into Kosovo and are holding their ground.

"The border is heavily mined, and the first week they had a lot of problems getting across there," said an international border monitor. "Now they're inside, and they're being shelled all the time from across the river near Vrbnica. But they seem to be hanging on."

Correspondents Peter Finn in Tirana and William Drozdiak in Brussels and staff writer Dana Priest in Washington contributed to this report.

CAPTION: Kosovo refugee Suna Shotri, 92, is helped to an aid station in Albanian border area near site of heavy fighting between Kosovo rebels and Yugoslav troops.