SARAJEVO -- Snipers opened fire on streetcars plying a Sarajevo thoroughfare Tuesday, after NATO jets flew over the Bosnian capital for a third consecutive night to quell fierce fighting in the city.


Civilians jumped out of streetcars when snipers, apparently Bosnian Serbs, opened fire, and sheltered behind two UN armored personnel carriers. An emergency medical team rushed to the scene but no casualties were reported.


The shooting followed a night in which government forces and besieging Serbs traded 175 mortar rounds in a duel that injured at least nine people and drew an overflight by NATO jets.


"Two F-15s flew overhead at around 20:05 GMT at the request of General Herve Gobilliard, Sarajevo Sector commander," Captain Miriam Sochacki, a UN spokeswoman, said. "Both sides heeded our requests to stop firing soon after."


The clashes began in the city's western suburbs, Sochacki said, with both sides using heavy weapons banned 20 kilometers from the center of Sarajevo under a NATO-backed UN resolution.


It was impossible to tell in the darkness who began the exchange of fire or what weapons were used, she said.


Witnesses reported shells landing around the city's television station, and blasts rocked tower blocks in areas near Sarajevo's airport.


It was the third night in a row that the presence of NATO jets has been required to restore calm to the city. A furious firefight Saturday night, judged the most serious threat to Sarajevo since September, ended when NATO jets appeared.


A further show of NATO strength was needed on Sunday evening to stem increasing tension between the Moslem-led Bosnian government army and Bosnian Serb forces in the Sarajevo area.


Serb forces have threatened to remove heavy weapons from UN control and deploy them against the Moslems unless they stop their offensives around the Bosnian capital.


With heavy fighting elsewhere bringing the Bosnian army some success and unnerving the Serbs, UN officials say the fragile February truce in the city could quickly unravel.


The Moslem-led Bosnian army and its Croat allies, pressing on with increasingly bold offensives, launched a blistering bombardment near the central town of Kupres on Monday.


Serb units were also reportedly defending access to Donji Vakuf, which UN peacekeepers expected to be one of the next targets for government troops waging an autumn offensive in northwest and central Bosnia and around Sarajevo.


Better armed and more tactically effective than ever, the Bosnians have attacked on several fronts, tying down Serb forces, which are short of men and fuel.


The Moslems took large amounts of ground in a surprise breakout from the Bihac pocket in the northwest, captured the central Bosnian town of Kupres, and are pressing the Serbs around the town of Trnovo, which lies south of Sarajevo.


The United Nations special envoy to the former Yugoslavia, Yasushi Akashi, said the Moslem offensive might change the military balance in the region and persuade the Serbs to reconsider their rejection of the Big Powers' peace plan.


Under the plan, the Serbs would be left with 49 percent of Bosnian territory, instead of the 70 percent they now control.


Akashi met UN Secretary General Boutros Boutros-Ghali to discuss the recent escalation of the conflict.


?The UN's Yugoslavia war crimes tribunal decided, in its first public hearing Tuesday, to ask Germany to hand over the case of a Bosnian Serb suspect who is now in its custody.


The tribunal, the first international war crimes panel since the Nuremberg trials in 1945, said it would ask Germany to continue to hold Dusan Tadic so that charges could be prepared against him.


Tadic, arrested in Munich in February, is suspected of killing, raping, beating and torturing Croats and Moslems during the "ethnic cleansing" of Bosnia's northwest Prijedor region.