WASHINGTON -- As NATO forces stepped up their missile assault into the daylight hours, President Slobodan Milosevic of Yugoslavia maintained fierce resistance Friday both on the ground and in the air.

Two of his MiG-29 fighters were shot down on what NATO said was a foray into neighboring Bosnia. The status of the pilots was unknown.The sweep by Serb troops through the ethnic Albanian villages of Kosovo continued unabated, NATO said, despite the third straight day of air assaults that NATO officials said were succeeding and would remain unrelenting until Milosevic accepted peace.

Unconfirmed reports of violence against civilians continued as refugees from the ethnic Albanian majority of Kosovo, Serbia's southernmost province, straggled across the border with Macedonia with tales of increasing anarchy.

There were also dramatic but unconfirmed reports of thousands of Kosovo refugees being pushed along a road with Serbian tanks at their back.

In the airstrikes, a series of explosions rocked the outskirts of Belgrade Friday night, with NATO officials saying the target list was being broadened specifi- cally to include bunker positions of the Yugoslav military hierarchy in the Belgrade area as well as army and police outposts in Kosovo.

NATO officials said even more severe assaults would follow.

President Clinton sought to appeal directly to the Serbian people in a televised statement, beamed by satellite throughout the region, in which he denounced Milosevic.

"He has diminished your country's standing, exposed you to violence and instability, and isolated you from the rest of Europe," Clinton declared in a broadcast translated into Serbo-Croatian.

In Belgrade, the government has claimed "minimal damage" from the NATO attacks in reporting at least a dozen people killed and scores wounded. But Milosevic's U.N. representative, Vladislav Jovanovic, said "hundreds of civilians" had been killed.

The downing of the two MiGs inside Bosnia by two U.S. F-15C fighters raised the question of whether Yugoslavia was attempting to extend the conflict and involve neighboring nations.

Administration officials said the planes were shot down from a four-plane group that violated Bosnian air space.

They crossed the western border of Serbia and were presumed to be heading aggressively toward NATO peace-keeping ground troops in Bosnia. The American interceptors were not from the NATO assault force but rather part of the Bosnia peace-keeping patrols.

The two losses, combined with earlier hits, means that by the Pentagon's count Milosevic has already lost a third of his premier air armada of 15 MiG-29s, highly rated Soviet fighter planes designed for both air-to-air and air-to-ground combat.

In a subsequent incident, two MiG-21s crossed over Bosnia but retreated before they could be intercepted, NATO said.

"It was an act of desperation," State Department spokesman James Rubin said of the MiGs' incursion, characterizing it as evidence of Milosevic's frustration at the NATO assaults.