SARAJEVO, Bosnia-Herzegovina (AP) _ NATO warplanes buzzed the Bosnian capital today after mortar fire rocked downtown areas and nearby front lines during Sarajevo's worst fighting in two years. At least one person, a 12-year-old girl, was killed.

Unusually heavy fighting was reported in at least five areas of Sarajevo, including near the National Theater and Old Town. Civil defense authorities ordered civilians off the streets and into shelters.

NATO planes flew low today over Sarajevo in a warning measure, but the United Nations said it would not call for air strikes in Bosnia's capital, besieged by Serbs since April 1992.

``What are you going to do, hit half of the city?'' asked Alexander Ivanko, a U.N. spokesman.

Ivanko appealed for a halt in the fighting, and said the Bosnian Serb commander, Gen. Ratko Mladic, promised his guns would try to avoid hitting civilians and U.N. peacekeepers.

But Bosnia Prime Minister Haris Silajdzic said his government was fed up with U.N. appeals and wanted action.

Several mortar explosions resounded every minute today in Sarajevo, along with heavy machine-gun and small-arms fire. U.N. officials recorded more than 800 explosions in seven hours of fighting, the worst in Sarajevo in two years.

U.N officials said up to 20 heavy weapons, most of them manned by the Serbs, were being fired from a so-called ``exclusion zone'' that extends 12 1/2 miles around Sarajevo and is supposed to be free of such arms. In one case, Serbs fired a mortar from a U.N. collection point for heavy weapons.

The Bosnian army accused its Serb foes of starting the fighting while Serbs charged the government launched an offensive south of the city to cut a Serb supply line.

A brother and sister were hit by shrapnel in the northern part of the city, hospital officials said. The girl, 12-year-old Azra Buljubasic, was dead upon arrival at a hospital. Doctors said her 17-year-old brother, Nedim, was severely wounded in the head and unlikely to survive.

In a related development, a 23-year-old French peacekeeper shot by a sniper in Sarajevo on Thursday died today of his head wound. He was the 37th French peacekeeper to be killed in Bosnia and Croatia since 1992.

Today's violence in the Bosnian capital was a sharp escalation of fighting that killed one and wounded four around Sarajevo late Sunday and Monday.

Outside Sarajevo, Serbs targeted the only road connecting the city to the outside over Mount Igman.

There was also fighting overnight in Bosnia's northwestern Bihac region. Fighting eased in the Orasje area to the northeast, site of three days of clashes between Bosnian Croats and Serbs trying to keep their corridor linking Bosnian and Croatian Serb holdings with Serbia proper.

Bosnian radio on Monday reported increased fighting elsewhere around the corridor, at Brcko, south of Orasje. Serbs were said to be using tanks and artillery against the mostly Muslim government troops.

Through much of the war, fighting has focused around the Serb supply route, which is less than three miles wide at Brcko. Cutting it there would isolate rebel Serbs both in Croatia and Bosnia from Serbia.

In neighboring Croatia, heavily armed Croatian troops and rebel Serbs faced off inside U.N.-monitored buffer zones, rendering the 13,000-strong U.N. force for Croatia largely powerless.

At the United Nations in New York, Croatian Ambassador Mario Nobilo promised Croat troops would withdraw from the zones within 24 hours.

But a U.N. spokesman in Zagreb, Lt. Col. Walt Natynczk, said there was no significant Croat withdrawal by midday today.

The confrontation maintained the tensions following a May 1 Croatian offensive that regained a chunk of territory won by Serbs in a six-month war in 1991.

The Croatian war left 10,000 people dead. The Bosnian conflict has been far worse, with 200,000 people dead or missing.