SARAJEVO, Bosnia-Herzegovina (AP) _ A Bosnian Serb leader today warned of new clashes in Sarajevo if plans to reunite the city under Muslim-led government rule are not postponed.

The warning highlighted Serbs' vehement resistance to the Bosnian peace accord that foresees Sarajevo's reunification by March 19. It came on the heels of the first meeting between rebel Serb officials and Muslim leaders in the government-held part of the city since the war began.

``There could be new clashes'' if the handover of Serb-held districts to the government is not postponed until Sept. 15, said Momcilo Krajisnik, speaker of the Bosnian Serbs' self-styled assembly, according to the Bosnian Serb news agency SRNA.

He said Wednesday's talks, mediated by the civilian administrator of the peace process, Carl Bildt, were ``useful,'' but ``could not be prolonged forever.''

Under the terms of the U.S.-brokered accord, four main Serb-held districts of Sarajevo are to come under government control. Threatening a mass exodus of residents from those districts, Serb leaders have repeatedly asked that the deadline be extended by six months.

If the transfer of power is not postponed, ``a certain number of Serbs will leave Sarajevo,'' Krajisnik said. ``Those who stay might organize armed resistance.''

Krajisnik said Serbs would like to stay in Sarajevo, ``but not under Muslim rule.''

Bildt called Wednesday's meeting ``historic'' and said mutual trust should be restored to try persuade Serbs to stay. But he ruled out any extension of the March deadline.

The meeting came only a day after an anti-tank rocket fired from a building in the Serb-held Grbavica district hit a streetcar, killing one woman and wounding 19 other people.

NATO officials dismissed Tuesday's rocket attack on a downtown streetcar as an isolated incident without official Bosnian Serb backing and rejected government calls for further punitive action.

It was the worst cease-fire violation since an Oct. 12 truce by Bosnia's warring factions that heralded the peace agreement signed last month in Paris.

``Our results show that the person firing the rocket shot in a vertical direction, up in the air, and not straight at the tram,'' Lt. Col. Richard Pernod, a spokesman for the NATO-led force, said today. ``We believe he was shooting to create terror, but not aiming at the streetcar.''

The Bosnian government claims NATO wants to play down the incident to avoid alienating Bosnian Serbs and jeopardizing their compliance with the peace accord. But Bosnian Foreign Minister Muhamed Sacirbey said that could have the opposite effect of encouraging further challenges to the accord.

Milorad Katic, head of the Serb-held area from where the rocket was fired, condemned the attack today as ``inhumane and barbaric.''

Serb authorities were doing everything to punish the culprit and ensure ``that nothing similar ever happens again,'' Katic said in a letter to the government.

Meanwhile, one of two U.S. soldiers injured Wednesday when their fuel tanker overturned in northeastern Bosnia was to be evacuated to Germany for treatment of injuries to his arms, said Maj. Peter Bullock of the NATO-led force.