KIGALI, Rwanda (AP) _ Tutsi rebels fired machine guns and mortar shells at government positions today in a renewed effort to take control of the capital despite a cease-fire pledge from both sides in the civil war.

The rebels and government forces also postponed truce talks for a day, said a U.N. military official who requested anonymity. The reasons for the delay of the talks planned for today were not clear, he said.

Leaders from the warring factions pledged Tuesday at the Organization of African Unity meeting in Tunis, Tunisia, to observe an immediate cease-fire. But the commitment has been slow in reaching soldiers fighting for control of Kigali, the capital.

The advancing Rwandan Patriotic Front rebels control most of the north and east of the country and now are trying to push the government soldiers out of the capital, which the provisional government has fled.

The fighting was less intense than during the past few days, the U.N. official said, though he had no casualty figures.

The United Nations tried today to resume evacuations of both Hutus and Tutsis trapped in Kigali but was again blocked by the factions, the U.N. official said.

The rebels, led by members of the minority Tutsi ethnic group, were trying to take control of key roads in the capital today, the official said. There was regular machine gun fire and sporadic shelling aimed at government forces.

The government army, dominated by the majority Hutu tribe, controls the city center but is largely surrounded by the rebels.

Since April, some 200,000 people have been killed in ethnic slaughter and in the war, although no reliable figures are available. The fighting broke out after the president, a Hutu, died in a mysterious plane crash while returning from a conference to solidify a tenuous peace between Hutus and Tutsis.

U.N. officials and rebels continue to find mass graves in the Kigali suburbs and Rwandan countryside.

Most victims are Tutsi civilians killed by government-trained Hutu militias.

Rebel officials in Kigali said they would not stop fighting unless the government stopped first.

''How can we have a cease-fire when the other side is attacking us? It makes no sense,'' Maj. Frank Kamanzi of the Patriotic Front said Wednesday.

The U.N. is particularly eager to move some 3,000 to 4,000 Tutsis holed up at the Ste. Famille church-school complex in a part of the city held by the Hutu government army.

The United Nations says up to 60 teen-agers were abducted Tuesday, and witnesses told U.N. officials the youths have been massacred. U.N. officials believe the reports but have been unable to confirm the killings because they have been denied access to the area where the teen-agers are believed buried.

France on Wednesday said it was ready to send peacekeeping troops if the cease-fire fails to take hold.

In an article in today's Liberation newspaper, Foreign Minister Alain Juppe added a call for trials for those responsible for the genocide in Rwanda.

''France ... demands that the leaders of this genocide be judged,'' Juppe wrote. They should be ''identified and excluded from all negotiations on the future of the country they helped destroy.''

But many Rwandans are strongly opposed to intervention by either Belgium, the colonial power, or France, which previously assisted the Hutu government.

''We think the French are responsible for what is happening in Rwanda, for the massacres, the genocide,'' said James Rwengo, the spokesman in Brussels for the rebels.

''France trained the army, the presidential guard, they even fought against the RPF (rebels) at one stage,'' Rwengo said. ''They should not participate in any force, even a United Nations one, much less carry out any action on their own.'' Hundreds of French and Belgian troops quit Rwanda after evacuating thousands of foreigners two months ago, early in the fighting. Ten Belgium troops were killed.