SARAJEVO, Bosnia-Herzegovina (AP) _ Aid planes to Sarajevo were grounded today after three U.N. aircraft were hit by gunfire, and Washington threatened to seek approval for arms sales to the outgunned Bosnian government if Serbs don't accept a peace plan by Oct. 15.

Unless the Serbs come round, Clinton plans to ask the U.N. Security Council to exempt Bosnia's Muslim-led government from a U.N. ban on arms sales to all factions in the former Yugoslav republic, administration sources said on condition of anonymity.

The vital airlift to Sarajevo was suspended just two days after it restarted because bullet holes were discovered once the U.N. planes returned safely to bases in Italy and Croatia, said Maj. Dacre Holloway, a U.N. military spokesman.

The planes were hit on the ground at the Sarajevo airport, said Peter Kessler, a U.N. aid spokesman. There was no immediate word on who was suspected of firing the shots.

The airlift, a lifeline for the besieged capital during most of the 28- month-old Bosnian war, was suspended more than three weeks ago after planes were hit by bullets believed to have been fired from Serb positions. It was restarted Tuesday.

''The situation in Sarajevo is extremely grave,'' Kessler said. ''We have almost no food stocks in the city.''

The shooting was another sign of mounting tensions around Sarajevo. Serbs have tried to tighten their siege of the city and Muslim-led government forces have gone on the offensive.

U.N. peacekeepers said as many as 3,000 Bosnian government troops have moved into a combat zone just north of Sarajevo in the past two days.

A peacekeeper spokesman, Maj. Rob Annink, said Bosnian Serb forces also were being strengthened in the volatile area, but he had no estimates of the number of their reinforcements.

''It is worrying,'' Annink said. ''Any increase in numbers can tip the balance.''

He said U.N. monitors had seen troops of the Muslim-led government army moving in buses and trucks through the town of Kiseljak, 15 miles northwest of Sarajevo, toward nearby front lines around the towns of Visoko and Breza.

The United Nations says some recent artillery duels in the area have extended into a 12.5-mile weapons exclusion zone around Sarajevo.

On Wednesday, the U.N. commander in Bosnia, Lt. Gen. Sir Michael Rose, warned both sides that NATO air strikes were a possibility if flagrant violations of the U.N.-mandated zone continued.

Government forces have been mounting an offensive along part of the front north of Sarajevo, apparently seeking control of a strategic road that eventually could serve as an access route into the capital.

Annink said, however, that it was not clear whether the Bosnian army troops or Serb reinforcements were intended for attack or defense. The Bosnian army confirmed its troop movements but gave no details.

Annink also reported that a Bosnian Serb T-84 tank remained in the Sarajevo exclusion zone today near Visoko despite a Serb commander's assurance that it would be removed.

Tanks are among the weapons that are supposed to be banned from the zone.

Since it was imposed last February, the ban has largely freed Sarajevo from intense bombardment. The city has been under Serb siege since the start of the war, which was launched by the Bosnian Serb minority in opposition to the republic's secession from Yugoslavia.

Rose has offered to deploy Canadian peacekeepers between the warring sides along the front north of Sarajevo. Asked whether this would prevent the government from seizing the strategic Sarajevo-Olovo route, Annink said: ''We do not secure this road for one party, but possibly for both.''

For a third straight day, the Serbs prohibited all U.N. peacekeeper convoy movements on Serb-held territory, Annink said.

In the northwestern government enclave of Bihac, U.N. officials were trying to defuse fighting between government forces and the last remaining soldiers loyal to Muslim secessionist leader Fikret Abdic.