WASHINGTON (AP) _ In the first spillover of NATO's air war with Yugoslavia beyond its borders, two U.S. Air Force F-15 fighters shot down two Yugoslav MiG-29 jets in Bosnia on Friday, officials said.

Pentagon spokesman Kenneth Bacon said it was not immediately clear why the Russian-built MiGs had crossed into Bosnian airspace, but he said it was possible they had intended to attack NATO peacekeeping troops in Bosnia.

``A second possibility is that they were involved in some sort of a Serb scheme to shoot down one of our planes,'' he said. ``And obviously if this was the scheme, it backfired egregiously on the Yugoslavs.''

The shootdowns brought to five the number of Yugoslav MiG-29s destroyed since NATO's air campaign began on Wednesday. Bacon said Yugoslavia has only 15 such planes, which are the best in its air force.

Contrary to initial reports by NATO officials, Bacon said the two MiG pilots had not been captured.

``We don't know if the pilots are alive,'' he said.

Bacon called the incident a ``serious challenge'' to NATO, but said the alliance had been prepared for possible efforts by the Yugoslavs to retaliate for NATO's severe airstrikes, which entered a third day Friday. As an extra security precaution, the American peacekeepers in Bosnia have been ordered to wear flak jackets whenever they are outdoors, even at their main base in Tuzla, Bacon said.

The F-15 fighters that shot down the MiGs were conducting routine air patrols over Bosnia as part of NATO's enforcement of a ``no-fly'' zone over Bosnia, a mission that has been going on since 1995 and is separate from the NATO air offensive against Yugoslavia. Bacon said it was not yet clear what kind of weapon the American jets used to bring down the MiGs, but they normally are armed with three kinds of air-to-air missiles.

The F-15s were based at Cervia, Italy, along the Adriatic coast, according to Cervia base public affairs officer Senior Master Sgt. Dale Warman.

One of the F-15 pilots, a U.S. Air Force captain, said in a telephone interview with The Associated Press after he returned to Cervia that he was surprised to find the Yugoslav warplanes in Bosnian airspace.

``It was two of us against what we thought was two of them,'' he said, speaking on condition of anonymity because he feared possible retaliation for his role. ``We actually saw their jets explode and go down.''

Bacon said the shootdowns happened five miles inside Bosnian airspace at 11:35 a.m. EST, or 5:35 p.m. in Bosnia.