SARAJEVO, Bosnia-Herzegovina (AP) _ A mortar shell apparently fired by rebel Serbs struck a tree at the U.N. headquarters in Sarajevo on Sunday, wounding three peacekeepers and the Bosnian police guard of the adjacent U.S. Embassy.

Seven more civilians were injured in other shelling, the Bosnian Health Ministry said. They included an Associated Press reporter and photographer wounded when two shells exploded right outside the U.N. compound 10 minutes after the first had hit a tree and sprayed both the U.N. buildings and U.S. embassy with shrapnel.

The attacks came during a weekend of escalating violence against civilians and peacekeepers in the besieged Bosnian capital. There are increasing questions about whether the U.N. can or should remain in Bosnia.

Thirteen people were killed and 75 wounded as Serb shells rained down on residential areas Saturday and early Sunday, the Health Ministry said. Streets were deserted, debris was scattered everywhere and the pavement was stained with blood.

Serb shelling, and targeting of U.N. facilities, have increased dramatically since the Muslim-led government launched an offensive June 15 to lift the 38-month siege of Sarajevo.

Around noon Sunday, a mortar shell detonated in the branches of a tree around the U.N. headquarters for Bosnia, said U.N. spokesman Lt. Col. Gary Coward. Three peacekeepers were injured by shrapnel, he said.

The Bosnian policeman guarding the adjacent U.S. Embassy compound, 27-year-old Sabahudin Luckin, also suffered shrapnel wounds to his hands, hospital officials said. Embassy staff took shelter in the basement.

``There are some very lucky people here,'' said Coward speaking from his office in the U.N. compound. Two of his windows were smashed by shrapnel, and a large piece of metal was embedded in a desk, he said.

``Given recent events, I believe it's reasonable to guess fairly accurately who fired,'' Coward said, referring to rebel Serbs.

AP photographer Santiago Lyon and AP reporter Srecko Latal both suffered shrapnel wounds in their legs when two more shells hit minutes later. Latal was released after hospital treatment, while Lyon underwent surgery to remove the shrapnel.

Earlier Sunday, French peacekeepers fired the biggest gun in their arsenal after Serbs attacked a U.N. convoy with an anti-aircraft battery on the sole, perilous route out of Sarajevo.

The peacekeepers on Mount Igman fired two shells from a 120-mm mortar, the biggest the U.N. has used since war erupted in Bosnia in April 1992.

They opened fire after a Serb battery of 30-mm anti-aircraft weapons attacked U.N. vehicles on the treacherous Mount Igman road, said Maj. Guy Vinet, a U.N. spokesman. No one in the convoy was hurt.

The Serbs failed to stop the attack after the peacekeepers fired a warning smoke shell, he said. The first shell landed 50 yards from the Serb battery, the second 20 yards away.

It was unclear if the Serb guns had been damaged, but Vinet said they stopped firing.

The French mortar had been deployed on direct orders from French President Jacques Chirac in an effort to better protect peacekeepers.

Elsewhere, Serbs said two boys, ages 11 and 13, were killed when Croatian troops shelled the Serb-held town of Brcko in northeastern Bosnia late Saturday. Four people were wounded, said the Bosnian Serb news agency SRNA.

There was no confirmation that the shelling originated in neighboring Croatia. The rebel Serbs have long feared that Croatian army troops would join forces with Bosnian Croats in the area.

Late Saturday, the U.N. aid agency reported a tragedy: the first hunger-related deaths in the northwestern government enclave of Bihac, which has seen only five aid convoys since April 5.

An elderly man was found dead in his apartment. He had left a letter saying he had nothing to eat and was too proud to beg. The second victim, a 3-year-old boy, weighed only 15 pounds when he died, the U.N. High Commissioner for Refugees said in a statement.