WASHINGTON (AP) _ The NATO plan to provide ``close air support'' to embattled troops on the ground in Bosnia could put allied pilots in the thick of one of the most difficult and dangerous operations in modern warfare.

The assignment involves using warplanes in coordination with ground troops to attack enemy forces _ in this case, Bosnian or U.N. soldiers under Bosnian Serb attack calling in NATO air strikes.

It raises the question of how effectively Bosnian troops would work with NATO pilots in the heat of battle, a crucial matter to ensure that the bombs land on the attacking forces, not the Bosnians or the peacekeepers.

``Close air support requires air-to-ground coordination of an intimate nature,'' said Richard Kugler, a senior defense analyst with the Rand Corp. who coordinated air strikes in Vietnam as an Air Force officer.

Such coordination could be difficult for forces that have never worked together, said both military and private analysts.

Defense Secretary William Perry and Gen. John Shalikashvili, chairman of the Joint Chiefs of Staff, outlined a scaled-back version Friday of what they had hoped would be a massive air campaign to stop Bosnian Serb aggression. Their comments came after intense meetings of the Western allies in London.

Instead of beginning with a massive campaign against Bosnian Serb air defenses and spreading to other military targets, the air campaign would focus on providing air support for Bosnian government troops on the ground in Gorazde as well as U.N. peacekeepers in the enclave.

If the situation continues to deteriorate, the air strikes could then broaden to include what Secretary of State Warren Christopher called ``significant attacks on significant targets.''

Possible targets of a broader air campaign could include:

_Air fields and air defenses at Banja Luka and an airfield at Udbina, across the border in territory held by the rebel Serbs in Croatia. NATO bombed Udbina last year but damaged only the airstrip, not the warplanes stationed there.

_Helicopter landing fields in Bijeljina and Zvornik.

_The road linking Pale and Sarajevo, a high priority of the Bosnian Serbs. When Bosnian government troops briefly cut off the link in a June offensive, Bosnian Serb forces moved quickly to regain control of the road.

_Artillery emplacements around Gorazde.

_Two weapons factories under Bosnian Serb control.

The main headquarters of the Bosnian Serb military, Han Pijesak in eastern Bosnia, is probably invulnerable to air attack; the bunker is buried in a mountainside and designed to withstand a nuclear blast.

``It first of all is a phased plan ranging everywhere from close air support for a particular tactical unit on the ground that is being attacked to a broader regional air campaign,'' Perry said at a London news conference. The profile of the strikes ``would be agreed to between the air commander and the ground commander and, in its latter phases, it involves an area considerably larger than Gorazde.''

Shalikashvili added that the strikes would encompass ``a wide range of targets throughout a broad zone of operations.''

Perry and Shalikashvili came to the London meetings with a plan to launch massive strikes against Bosnian Serb air defenses and to broaden the campaign to other targets. They acknowledged that their plan posed the risk that the Bosnian Serbs would once again respond by taking U.N. peacekeepers hostage.

A U.S. defense official familiar with the latest intelligence on Bosnia said a Bosnian Serb takeover of all the safe areas would not only expose the Muslim inhabitants to expulsion and severe abuse at the hands of the rebel Serbs, but would also jeopardize the survival of the main central Bosnian territory held by the Bosnian government.

In Gorazde, the Bosnian government has 10,000 troops against 5,000 Bosnian Serb forces around the enclave, said the defense official, who spoke on condition of anonymity. Thus a key goal of the air campaign would be to intercept Bosnian Serb reinforcements by striking road links. U.S. intelligence officials estimate the Bosnian Serbs would be unable to mount a credible assault on Gorazde for ``a couple of weeks.''

Col. Andrew Duncan, assistant director of the London-based International Institute for Strategic Studies, questioned whether tactical strikes could turn the tide.

``Air power is a very temporary way of stopping something,'' said Duncan, a retired British army officer. ``Resolute people can do a lot between the raids.''