SARAJEVO, Bosnia-Herzegovina (AP) _ Officials suspended Sarajevo's airlift today because of gunfire that hit a U.N. plane.

Rebel Serbs revoked permission for food shipments to reach hungry people in northwest Bosnia, and peacekeepers complained about increasing Serb obstructions of U.N. operations.

The U.N. plane was the fourth struck by small-arms fire in six days at Sarajevo airport, an indication of rising tensions in the capital and across Bosnia. A four-month truce, now at its halfway mark, has been violated dozens of times, particularly in the northwest.

The airlift to the Bosnian capital was halted for both military and relief planes after an Ilyushin 76 aircraft belonging to the U.N. force was hit by five bullets while taxiing on the runway this morning, said U.N. military spokesman Maj. Herve Gourmelon. It was unclear when the flights would be resumed. No one was injured.

Another Ilyushin 76 was hit on Wednesday.

Officials also reported heavy fighting for a third day close to the renegade Muslim stronghold of Velika Kladusa in Bosnia's volatile northwestern region.

Kris Janowski, a spokesman for the U.N. High Commissioner for Refugees in Sarajevo, said permission for desperately needed aid convoys to cross Croatian Serb-held territory and reach the besieged Bihac pocket had been canceled by Serb authorities.

``We are back to totaly unacceptable levels of supplies for the people of Bihac,'' Janowski said.

A resupply convoy for the U.N. Bangladeshi battalion was unable to enter the pocket due to fighting Wednesday and had to turn back to Zagreb, said Chris Gunnes, a U.N. spokesman in Zagreb.

Only one of five planned relief convoys this week reached the government-held part of the northwestern Muslim enclave.

Janowski said the Serbs broke an earlier pledge, obtained in part thanks to the personal intervention of U.N. special envoy to Yugoslavia Yasushi Akashi, to allow convoys safe passage to Bihac.

Aid convoys scheduled to leave Zagreb, the Croatian capital, today and Friday have been called off. Meantime, Tuesday's food convoy, which has been stranded close to Croat-Bosnian border for two days, will return to Zagreb, Janowski said.

Only one convoy, carrying around 90 tons of food reached Bihac this week. U.N. spokesman Lt. Col. Gary Coward said today that the logistic and medical supply situation for U.N. peacekeepers in eastern Muslim enclaves was also deteriorating because of dwindling supplies.

He also reported that Bosnian Serbs did not allow a high-level U.N. military visit to the eastern enclave of Zepa on Wednesday.

Red Cross officials in Sarajevo said Serbs have failed for four weeks to give the go-ahead for a relief convoy from Zenica to go to Sarajevo.

A Western diplomat, speaking on condition of anonymity, said today that a key reason for cancelling all UNHCR convoys to Sarajevo next week was the Serbs' demand for an increased share in aid. They currently receive about one-quarter of the total.

Meanwhile, U.N. military observers were barred from investigating a mortar suspected to be in the Serb-held part of Dobrinja, a southern Sarajevo suburb, in violation of an existing exclusion zone, Coward said. The same applies to four other heavy weapons believed in Serb-held Rajlovac north of the capital.

Fighting persisted in the Bihac enclave. More than 800 detonations were recorded Wednesday along front-lines south and southeast of Velika Kladusa, and 342 blasts this morning, Gourmelon said.