U.N. Ponders Role as Fighting Sweeps Across Sarajevo : Bosnia: As clashes intensify, world body weighs proposals to alter its failing mission.

ZAGREB, Croatia —  Foreshadowing a long-feared showdown between Balkan enemies, the fiercest fighting in two years on Tuesday rattled Sarajevo, the capital of Bosnia-Herzegovina, while U.N. peacekeepers contemplated an overhaul of their failing mission.

Rebel Serb and government soldiers fought hand-to-hand in trenches on the edge of Sarajevo, U.N. military officials said, and hundreds of mortars, rockets and heavy artillery rounds pounded the city through most of the day.

Residents, who had found relief from this sort of terror during a four-month truce that ended May 1, fled through the streets and took shelter in basements as shells exploded around them.

North Atlantic Treaty Organization warplanes buzzed the city, but a Bosnian government plea for air strikes was denied.

“Once again, the parties have opted for a military solution,” U.N. spokesman Alexander Ivanko said in Sarajevo. “Once again, it is the civilians who become the ultimate victims of this tragic war.”

At least two people were killed, and 11 civilians and two U.N. peacekeepers were wounded, Ivanko said in a telephone interview from Sarajevo.

Later news reports put the death toll at five, with nearly 30 people hurt.

The fighting started when Bosnian government troops lobbed four mortars into a Bosnian Serb military base just outside the city and at the foot of a Bosnian Serb supply road that leads to the rebels’ mountaintop stronghold of Pale, U.N. officials said.

The Muslim-led government, however, blamed the Bosnian Serbs for starting the fight.

The subsequent action fit the recent pattern of an increasingly aggressive Bosnian army, U.N. military analysts said.

Since March, government forces have struck at Bosnian Serb positions in limited but effective attacks that force the Serbian rebels to expend resources and equipment at a time when they are already stretched thin to protect the 70% of Bosnian territory they seized in three years of war.

Having used the last year to build, train and equip its once-weak army, the Bosnian government has threatened to launch an offensive that would break through the siege by the Serbs, who have cut off Sarajevo.

Most analysts thought Tuesday’s fighting was not the start of such an offensive but one of what will be a series of escalating clashes until an all-out confrontation this summer.

In Tuesday’s battles, Bosnian Serbs fired heavy weapons from the exclusion zone around Sarajevo, where such arms are supposed to be banned under a U.N.-brokered agreement.

Bosnian Serbs have been steadily moving their heavy artillery back into the area in violation of the agreement; at one point Tuesday, they opened fire on Sarajevo from a U.N. collection point for heavy weapons.

Although fighting had subsided by nightfall, the United Nations again appeared as helpless to influence events as it has throughout its troubled three-year stint in Bosnia.

Tuesday’s flare-up came as U.N. officials outlined proposals to radically alter their efforts.

Each day, it becomes clearer that the United Nations no longer has control over the warring parties in the former Yugoslav republics. And each day, world capitals that support the mission with people and money seem less willing to continue that support.

U.N. officials here argue that if their troops are not given more firepower and leeway in engaging the factions, they might as well scale down to a largely symbolic monitoring function. Some even suggest that the U.N. presence has done nothing more than delay a nasty but inevitable conclusion that would give much of Bosnia to the Bosnian Serbs and leave civilians to their mercy.

Officials drafted a 15-page report for U.N. Secretary General Boutros Boutros-Ghali.

At U.N. headquarters in New York, he told reporters that his aides were preparing proposals aimed at protecting the peacekeepers to prevent the collapse of the Bosnian mission. He said he leaned toward a reduction and redeployment of troops in “safe areas” of Bosnia but refused to say if he intended to pull them out of any of these towns.

He said he planned on presenting options to the Security Council in the next few days, depending on advice from aides. These included peace enforcement, a sustainable air strike (what would amount to “military confrontation”) and redeployment and reduction in the “safe areas” of Srbrenica, Tuzla, Zepa, Gorazde, Bihac and Sarajevo.

It will be up to the council, he said, to choose an option.

Times staff writer Stanley Meisler in Washington contributed to this report.