ZAGREB, Croatia (AP) _ Serb fighter planes have used U.N. relief flights as cover for sorties over Bosnia-Herzegovina, a senior Croatian military official claimed today.

When the Serb-dominated Yugoslav federal army withdrew from breakaway Bosnia-Herzegovina in June, it left behind an estimated 50 aircraft with Bosnian Serb forces fighting Croats and Muslims, military analysts said.

Late last month, Serb fighters were detected ''flying in a radar mask'' of U.N. flights ferrying food and medicine from the Croatian capital, Zagreb, to the Bosnian capital, Sarajevo, Col. Milan Macek, chief of staff of the Croatian Air Force, told The Associated Press.

During that time, he said, there was ''intensified fighting'' by Serb planes. But Macek offered no proof that that fighting was related to the alleged shadowing operation.

The Serb aircraft were following the international humanitarian flights in the hope they could elude ground radar, Macek said. He refused to say how many planes allegedly used the relief corridor for cover and claimed not to know whether they fired or were fired upon.

Serb officials were not immediately available for comment on the allegations.

There have been repeated reports recently of Serb planes bombarding Bosnian government strongholds.

Macek claimed that on one occasion, Croatian radar control detected two Serb fighters flying together in the radar shadow of a U.N. relief plane.

Croatian officials have informed U.N. officials in Croatia about the alleged shadowing, Macek said.

It was possible the U.N. pilots did not see the planes following them, he said.

U.N. officials in Zagreb could not immediately be reached for comment.

A British relief aircraft flying out of Sarajevo was tracked by unidentified ground radar last month, forcing the airport to be shut down. More recently, a French aircraft reported it was tracked by airborne radar from a fighter, but the radar's source was not identified, said a diplomat at the United Nations in New York. He spoke on condition of anonymity.

Last week, an Italian relief plane crashed on its descent into Sarajevo, and all four crewmembers were killed. Investigators say they believe it was hit by at least one ground-to-air missile.

The New York Times reported from Washington in its Friday editions on the allegations of Serb shadowing. The report said some government officials speculated that the alleged practice may have contributed to the downing of the Italian plane. The U.S. Defense Department said the Italian plane was not being shadowed, the newspaper said.

American officials were not immediately available for comment today.

France has been pushing the U.N. Security Council to ban military flights by forces of the former Yugoslavia.

DT