SARAJEVO, Bosnia-Herzegovina (AP) _ Bosnian Serbs shelled a Sarajevo suburb for the second straight day today, closed all checkpoints into Sarajevo and threatened to shoot down U.N. helicopters flying over their territory.

The rising tension between Serbs and the United Nations followed a request Thursday night by French peacekeepers near Sarajevo for two NATO warplanes to buzz Serb positions in a show of force.

Bosnia's battlefronts continue to deteriorate in the vacuum of stalled peace talks. All sides have refused to extend a cease-fire, which while practically nonexistent, continues until May 1.

Sarajevo and its southwestern suburb of Hrasnica were under alert after a night and morning of intermittent Serb shelling, with residents being urged to stay indoors. On Thursday, four people were killed and 10 wounded in the shelling, state radio said.

The United Nations said it had unconfirmed reports that Bosnian Serbs had fired a rocket into the center of Hrasnica, a government enclave that lies between the U.N.-controlled airport and strategic Mount Igman.

Bosnian Serbs, who surround Sarajevo, closed all checkpoints into the capital today in a dispute with peacekeepers. The United Nations had brought in reinforcements, including three armored personnel carriers mounted with 20 mm guns, to monitor a troublesome Serb checkpoint near the airport.

Serbs ``have to stop hijacking our vehicles,'' said U.N. spokeswoman Capt. Myriam Sochacki. ``They have to respect freedom of movement on that road.''

Bosnian Serbs warned they would shoot at the APCs if they were not withdrawn, she said. Serbs reinforced their troops at the checkpoint to some 15 armed soldiers.

``We don't want to use force, but if they (the APCs) are directly threatened, they will defend themselves,'' Sochacki said.

A U.N. source, speaking on condition of anonymity, said Serbs at the checkpoint have seized 15 vehicles, two anti-tank weapons, one heavy machine gun, numerous rounds of ammunition, 11 walkie-talkies and 130 flak jackets from the United Nations this year.

Several journalists and aid workers have also been seized, and a Bosnian vice president was shot and killed when his U.N. transporter was stopped there in 1993.

Angered that a helicopter carrying a top U.N. commander, Lt. Gen. Bernard Janvier, did not seek clearance to land in the northwestern Bihac region, Serbs also threatened to shoot down any U.N. helicopter flying over their territory, said Maj. Herve Gourmelon, a U.N. spokesman.

State radio said one person was killed and 10 wounded Thursday in Serb shelling of the Bihac enclave. U.N. officials confirmed the shelling but could not verify the casualties.

In another U.N.-Serb showdown, peacekeepers manning an observation post on Mount Igman exchanged heavy weapons fire with Serbs for several hours Thursday night, and called in two NATO warplanes to buzz the area.

The peacekeepers were trying to halt attacks on the only civilian access route into Sarajevo, which crosses Mount Igman. Serbs kept firing even after the NATO show of force, targeting the U.N. post with two mortar shells, which landed nearby.

In the northeast around Tuzla, Bosnian Serbs claimed to have repulsed a government offensive on a strategic communications relay tower.

An estimated 200,000 people have died or disappeared since war began in April 1992 with a Serb revolt against Bosnia decision to become independent from Serb-led Yugoslavia.