ZENICA, Bosnia-Herzegovina (AP) _ Officially, the Bosnian government air force isn't allowed to fly its own skies. But this improvised, low-tech unit makes its own rules.

NATO warplanes, enforcing a no-fly zone that applies to both warring sides in Bosnia, don't go after helicopters from either side because of the risk of civilian casualties.

The Bosnian Serbs aren't so lenient.

Pilot Mustafa Kahric recalled flying just above the treetops in an outdated Mi-8 Hip helicopter, dodging cliffs and Serb anti-aircraft fire in central Bosnia.

``The day after, Serbian TV reported that a Muslim helicopter was destroyed,'' said Kahric, 39, a brigade commander who says he's made 11 flights to besieged government enclaves. ``But I'm still alive and flying.''

Outside his office, soldiers and technicians were refueling and preparing helicopters for new missions.

U.N. officials have repeatedly protested helicopter flights by both the government and rebel Serb sides as violations of the no-fly zone. But Bosnian pilots say there appears to be a tacit agreement by NATO pilots to let them be.

``They know we are usually unarmed and transporting medicines, supplies or casualties,'' said Bosnian air force Capt. Salih Jamakovic. ``So they don't shoot us down.''

Other government flights, however, fly in ammunition, high-tech weapons or special forces troops. Sometimes, top Bosnian political or military officials are on board to visit soldiers in areas outside Sarajevo.

Serb helicopters also are tacitly allowed to fly despite U.N. suggestions that they are resupplying troops. Four Bosnian Serb warplanes were shot down in 1994 for violating the no-fly ban in central Bosnia.

The Bosnian air force was started with small single-engine recreational planes, dilapidated helicopters and a few former Yugoslav army pilots loyal to the Bosnian government.

Col. Erdin Hrustic, the air force chief of staff and a former jet fighter pilot in the Yugoslav air force, recalled the fledgling air force's grim baptism of fire in June 1992, the first year of the war.

During an attempt to evacuate a casualty from Bosnia to Zagreb, Croatia, the small plane had engine trouble and made a forced landing. The pilot was badly hurt and the injured passenger died.

The air force has grown steadily since. Several aircraft, mostly helicopters, have been bought. Several air bases have been established, and an unknown number of fixed-wing aircraft also are operational.

Most of the aircraft are old Russian transport helicopters, but army sources say the government plans to buy more helicopters, including fighter models.

By comparison, military analysts believe the Bosnian Serbs have about 20 helicopters and 25 planes, including combat and light support aircraft.

The Bosnian helicopters fly regularly to besieged enclaves such as Gorazde and Bihac to deliver supplies and evacuate critically injured people, conducting the missions at low altitudes after dark to avoid Serb radar and fire.

Some pilots claim to have flown directly over Pale, the Bosnian Serb stronghold east of Sarajevo.

``When you finally get to Bihac or Gorazde with medicines, supplies or to pick up injured, you see euphoric faces of local people,'' Kahric said. ``We are the proof that they are not completely forgot and cut off. Their faces are the best possible reward for us.''