SARAJEVO, Bosnia-Herzegovina (AP) _ The U.N. peacekeepers' experience in what was Yugoslavia, often humiliating, suggests that even a beefed-up force may not be able to protect ''safe areas,'' U.N. officials say.

Despite instances of heroism, the U.N. Protection Force, or UNPROFOR, is already known as ''Unprofessional Force'' by some officers who criticize uneven troop quality and poor coordination. Incidents of corruption among some U.N. soldiers could also undermine the forces' credibility among local civilians.

Serb contempt for the peacekeepers runs so high, one U.N. official said on condition of anonymity, that soldiers at a roadblock opened up one Ukrainian armored vehicle and urinated inside. The Ukrainians drove off, leaving a food convoy behind.

At Mostar, northwest of Sarajevo, Croatian troops harassed the retreat of a Spanish convoy by placing mines under its personnel carriers as they backed away in reverse. None exploded.

The U.N. peacekeepers have also been hampered by political contraints imposed by the U.N. Security Council. They may only fire their weapons in self-defense, or to defend their missions, such as escorting convoys of humanitarian aid or fleeing refugees.

This has led to situations in which peacekeepers have observed Serbs rounding up Muslims for deportation from villages as part of an ''ethnic cleansing'' campaign, but are powerless to do anything to stop them.

''In peacekeeping, without credibility you are dead,'' said Cedric Thornberry, chief U.N. civil affairs officer, in Zagreb, the Croatian capital. ''Military presence must be paralleled by political support.''

He avoided specific criticisms, but others said that the slow pace of peace talks, along with lukewarm enthusiasm for intervention in the West, blunted the U.N.'s impact.

Field officers said they doubted that a larger operation could protect safe areas from guerrilla tactics unless local Serb and Croat commanders feared counterattacks.

That is unlikely unless U.N. forces receive a mandate to be much more aggressive militarily. Right now their mandate is to escort aid convoys, oversee refugee evacuations, and protect besieged civilians.

UNPROFOR's 25,000 troops and civilians, from dozens of nations, include such crack units as Britain's Prince of Wales's Own Regiment, stationed at Vitez in Bosnia.

Except for larger British and French contingents, most countries have sent battalions, usually about 900 men. All face similar frustrations.

''We could shoot up a roadblock,'' said Maj. Charles Guermeuil of the French Foreign Legion. ''But we're the U.N. here. It's complex. The ones who would suffer are the civilian truckers behind us.''

As a result, convoys turn back whenever challenged.

Muslim leaders say they have lost faith in the United Nations which, they say, has allowed Serbs and Croats to block relief shipments to some places for up to two months.

The vital Tuzla-Zenica area of central Bosnia was closed to commercial traffic and nearly all relief convoys during April and most of May.

''They don't want a clear mandate to open the roads,'' Ejup Ganic, vice president of Bosnia-Herzegovina, said of the U.N. forces. ''They like to negotiate with every idiot. They want these hopeless situations to prove how hard they are working.''

U.N. officials deny this but admit they are often hamstrung and humiliated by Serb and Croat commanders who use hunger as a weapon.

Occasionally, commanders stretch the limit of their mandate and place their units between attacking forces and beleaguered civilians.

French Gen. Philippe Morillon, head of U.N. forces in Bosnia-Herzegovina, remained in Srebrenica in March until Serbs stopped shelling.

More often, U.N. troops watch helplessly.

In April, a small band of Serbs swept past Nigerian troops into the Bihac pocket, a Muslim enclave thrusting into a Serb-dominated fringe of Croatia. They seized eight square miles (13 square kilometers) of territory.

''If we had known they were coming, we would have gotten in their way,'' said Lt. Col. Luke Apresi, commander of the Nigerian battalion at Glina. ''But they sneaked in at night in civilian vehicles.''

In fact, other U.N. officers said, the Serbs used military vehicles and simply rolled past dispirited, ill-equipped Nigerian troops who had not been paid in five months.

One Nigerian soldier, interviewed as he was selling black market laundry powder to a Croat woman at a U.N. canteen, said the troops had no interest in fighting. In May, he said, they threatened mutiny.

Nigerians said their government decided they would get only a few dollars a day of special duty allowance, less than 5 percent of some other troops' U.N. pay, and even that was mysteriously withheld.

Apresi said his government reduced the allowances because the U.N. scale was out of line with wages back home. An administrative officer, who asked not to be named, confirmed the late payment.

Thornberry blamed some problems on late contributions by U.N. members but had no details on the Nigerians' case.

The Nigerian battalion was so badly supplied, other U.N. officials said, that one soldier froze to death at a checkpoint. Two others were murdered by Serbs whom they let approach unchallenged.

Around Sarajevo, Ukrainian troops have run a thriving black market, selling fuel and other supplies, according to U.N. officers and customers. Diesel cans on the market bear Ukrainian markings.

On one occasion, a Ukrainian convoy was sent through Serb lines to nearby Kiseljak to bring back workers for the U.N. High Commissioner for Refugees and three Roman Catholic nuns.

The Ukrainian soldiers filled their armored personal carrier with so much black market booty that one of the nuns had to be left behind, said a senior UNHCR official, who asked for anonymity.

Other U.N. officials and French sources reported cases of Egyptian troops selling their weapons.

Coordination is often weak and bureaucratic procedures imposed from headquarters in New York discourage initiative, officers said. Units are scrupulous about avoiding precedents or taking sides.

After two young lovers were shot trying to flee Sarajevo in May, their bodies lay seven days in no man's land until Serbs finally retrieved them.

''Where was the U.N.?'' demanded Milkan Gojkovic, a friend, at their burial. ''Why don't they just go home?''

Later, a U.N. official explained that troops could have gone to get the lovers in an armored vehicle, but that might set a precedent: they would have to recover all bodies.