SARAJEVO, Bosnia-Herzegovina (AP) _ Pushed by Bosnia's combatants to choose between getting tough and getting out, the United Nations on Friday told its beleaguered peacekeepers to shoot to kill to protect themselves.

Although all peacekeeping missions are told they can return fire to protect themselves, troops often withhold fire depending on how assertive the United Nations is trying to be in protecting its mission. Friday's blunt instructions signaled that peacekeeping nations were likely to stay.

France in particular had been threatening to withdraw unless the mission took a stronger posture. The instructions were issued a day after a sniper shot a French soldier in the head near Sarajevo's notorious ``Sniper Alley.''

A withdrawal of the 24,000 U.N. soldiers in Bosnia almost certainly would draw in the NATO alliance to protect them while they moved out. It also would lead to a scramble by Bosnia's armies for U.N. military equipment and to heavier fighting.

In response to the new instructions, a U.N. crew fired a single rifle shot after a Serb anti-aircraft round struck near their armored vehicle by the Jewish cemetery, a front line where there was heavy fighting Friday.

Bosnian Serbs said two women were killed by government snipers in Serb-held Lukavica. Serb shelling killed a woman in a northern Sarajevo suburb, and snipers wounded a girl in the western suburb of Dobrinja.

Aid officials meanwhile reported the northwest corner of Bosnia, where government forces and 200,000 civilians are surrounded, was facing catastrophic food shortages. A Red Cross convoy reached the area Thursday, but the U.N. High Commissioner for Refugees has not been there with food since April 5.

Aid problems occurred as the U.N. commander in Bosnia, British Lt. Gen. Rupert Smith, reminded his troops that they ``should not hesitate to use force when targeted by either warring party,'' said spokesman Lt. Col. Gary Coward.

``Soldiers shoot to kill,'' Coward said. ``I think we are doing the right thing at the right time.''

Smith's message to his soldiers was bolstered by the five-nation Contact Group _ the United States, Russia, Britain, France and Germany _ which has been leading the effort to find a solution to the 3-year-old Bosnian war that has left 200,000 people dead or missing.

Meeting in Frankfurt, diplomats from those countries called for a ``robust implementation of the U.N. mandate.''

After a meeting in Paris at which U.N. officials requested clearer instructions on how to respond to attacks, U.N. Secretary-General Boutros Boutros-Ghali ordered a ``fundamental review'' of their mission.

The U.N. mission in Bosnia is complicated by vague Security Council resolutions, a lack of military muscle to accomplish its mission, and the need to temper its military responses to further peace negotiations.

When the United Nations called in NATO airstrikes against Serbs last fall, the Serbs responded by detaining hundreds of peacekeepers. On Monday, U.N. officials decided against calling in air power to punish Serbs for a mortar attack on a Sarajevo suburb that killed 10 people.

U.N. spokesman Alexander Ivanko in Sarajevo said Friday that attacks on peacekeepers appeared well-coordinated.

``There are too many of them, too often,'' Ivanko said. Although peacekeepers cannot always determine who is firing at them, most appear to be from the Serb side, he said.

France, with the largest contingent of peacekeepers in Bosnia, also has suffered the heaviest losses _ 36 soldiers killed. Last month, two French peacekeepers in Sarajevo were shot and killed by snipers on one weekend, prompting the government to announce it was considering pulling out.

French Foreign Minister Alain Juppe discussed the possibility of allowing peacekeepers to use more force with Boutros-Ghali on Thursday.

Concern about limiting the Bosnian war has been particularly high in recent weeks as the civil war in neighboring Croatia threatened to reignite.

The Croats last week retook a piece of land known as western Slavonia, the most vulnerable of four Croatian regions seized by Serbs in the 1991 Croatian war.

U.N. spokesman Yuri Chizhik said U.N. officials were working in the area, but that in some places Croatian police were restricting their movements.

To the south, in Serb-held Bosnia, the Red Cross said a Roman Catholic church near the Serb stronghold of Banja Luka had been blown up, and that a priest and nun were reported missing.

Croats are traditionally Roman Catholic, while Serbs are Christian Orthodox. Bosnian Serb fighters blew up two Catholic churches Sunday.