NATO's growing armada of warplanes dominates the skies over Yugoslavia, but only down to a few hundred feet off the ground.
Below that altitude, a small but deadly fleet of Yugoslav helicopter gunships and ground-attack planes is waging a treetop air war, taking advantage of the hilly, wooded terrain and NATO's focus elsewhere to terrorize ethnic Albanians and blast rebel fighters in Kosovo.
NATO officials say that to effectively ground Yugoslavia's low fliers would require diverting scores of planes from a campaign that has already been criticized by some Air Force generals for not launching larger waves of bombing runs and hitting more targets.
Moreover, NATO jets would have to fly much lower to target the Yugoslav planes, putting allied pilots at greater risk from ground fire and surface-to-air missiles.
So while refugees streaming into Albania have reported strafings or bombing by low-flying green planes -- the color of most of Yugoslavia's aircraft -- NATO commanders say the threat does not justify dramatically altering the tactics of their air campaign.
''If they succeed in taking off, their mission has to be short, has to be of limited size and with limited scope,'' NATO's military spokesman, Brig. Gen. Giuseppe Marani of Italy, said earlier this week in dismissing the significance of Yugoslavia's Air Force.
While NATO is not ignoring this air threat, its main goal is to attack armored forces and special police in the field, as well as the supply lines and command bunkers north of Kosovo that support them. NATO has also put a high priority on destroying Yugoslavia's top-of-the-line MIG-29 and MIG-21 fighters, which pose a direct threat to allied warplanes.
Four other Yugoslav aircraft can take off and land quickly on remote landing strips, and cruise low enough to duck under the gaze of Awacs radar planes: the Galeb and Super Galeb attack planes and the Gazelle and Hind attack helicopters. These aircraft can also operate effectively under the same clouds and bad weather that have thwarted scores of NATO warplanes flying above 15,000 feet.
The Super Galebs, in particular, have effectively attacked mountain camp strongholds of the Kosovo Liberation Army. The light attack planes are equipped with 23-millimeter guns and antipersonnel cluster bombs.
Earlier this week, NATO reported an increase in attacks by Yugoslav planes and helicopters, including many on Sunday. Allied jets were not close enough to strike the Yugoslav aircraft, NATO commanders said. But American military intelligence said today that the low-flying aircraft had curtailed their missions as the week went on.
The Pentagon estimates that allied jets have destroyed nearly three dozen of Belgrade's lighter fighter-bombers.
''We have reduced by probably around 50 percent his airborne ability to conduct ground-attack operations,'' Rear Adm. Thomas R. Wilson, the director of intelligence for the Joint Chiefs of Staff, told reporters on Thursday.
Many of the ground-attack planes had been based at airfields in Nis, in Serbia, and in Podgorica, in Montenegro, Admiral Wilson said.
Because all of Yugoslavia is a combat area, allied jets are, in effect, enforcing a no-flight zone over the entire country. But the scope of this air campaign is much broader than the one that American-led patrols are enforcing over northern and southern Iraq. The allies created no-flight zones in those two areas after the 1991 Persian Gulf war to prevent President Saddam Hussein from attacking Kurds in the north and Shiite Muslims in the south.
In Yugoslavia, allied pilots are attacking scores of targets every day, from fuel depots and ammunition dumps to the armored ground forces that are expelling thousands of Albanians from Kosovo.
In contrast, the main mission of pilots enforcing the no-flight zones over Iraq is to prevent Iraqi aircraft from flying. It seems to have worked: the threat of being shot has largely grounded Baghdad's Air Force. In fact, until Iraq recently stepped up a belligerent campaign of targeting allied planes with its surface-to-air missiles, United States pilots complained of the tedious duty of ''boring holes in the sky.''
If NATO and Yugoslavia ever negotiate a settlement over Kosovo, the allies are not likely to repeat the mistake Gen. H. Norman Schwarzkopf made at the armistice meeting to end the gulf war. At that meeting, General Schwarzkopf agreed to Iraq's request to be allowed to fly armed helicopters anywhere inside Iraq so long as they were not near American forces.
Iraq used that air power, which it claimed was necessary for self-defense, combined with heavy ground armor that survived the war, to kill thousands of rebellious Kurds and Shiites.