Clashes Halt U.N.’s Sarajevo Aid Flights : Balkans: Western governments disclose critical letter adding steam to drive for ouster of the Serbian president.

A new eruption of ethnic fighting Wednesday in Bosnia-Herzegovina forced the United Nations to indefinitely suspend relief flights to Sarajevo, worsening what is already a life-threatening shortage of food as winter descends on the battered republic.

Meanwhile, in this tense Serbian and Yugoslav capital, Western governments added steam to the burgeoning movement to oust Serbian President Slobodan Milosevic by publicly disclosing a letter accusing him of doing nothing to halt the Bosnian bloodshed.

The Office of the U.N. High Commissioner for Refugees in Geneva blamed the interruption of humanitarian aid flights on fierce clashes near the Sarajevo airport and in the town of Vitez, about 40 miles north of the Bosnian capital.

Croatian Radio said the fighting in Vitez was between Croats and Muslim Slavs, who until recently were allied against Serbian rebels fighting Bosnia’s proclaimed independence from Yugoslavia.

U.N. officials have been warning for months that hundreds of thousands uprooted by the fighting--mostly Muslim civilians--could die of starvation or exposure this winter if the war in Bosnia continues to block delivery of aid. Relief shipments by road were cut off last week after two convoys from the Croatian port of Split were shelled en route to Sarajevo.

“This is just one more nail in the coffin for the people we’re trying to help,” U.N. refugee agency spokesman Ron Redmond told reporters in Geneva after announcing that the airlift had been halted.

In Washington, the Bush Administration, which recently has blamed the Serbs for most of Bosnia’s ethnic violence, condemned the outbreak of Croat-Muslim fighting.

“Those perpetuating the fighting should understand the suffering that it causes,” State Department spokesman Richard Boucher said. “By making it impossible to deliver relief supplies, the Croats and the Muslims in this instance bear the responsibility for the increased hardship and deprivation faced by the citizens of Sarajevo.”

The escalation of ethnic fighting follows the announcement a day earlier that Bosnian President Alija Izetbegovic had given up efforts to retain a unified republic, agreeing to its division among the warring factions.

That concession probably sparked the intensified clashes around Sarajevo and Vitez as all three ethnic groups rushed to secure more territory in advance of the impending division.

Bosnian Serbs control 70% of the republic in which they made up less than one-third of the 4.4 million population before war broke out in April.

Croatian forces hold 25% of Bosnian territory, mostly in the southwestern Herzegovina region where Croats predominate.

That has left little room for the Muslims, who were 44% of the prewar population. Their numbers have been depleted, since they account for most of the 14,000 killed and 50,000 missing in the war, as well as the bulk of nearly 2 million driven from their homes by fighting and “ethnic cleansing.”

The letter sent to Milosevic by Western embassies, including the United States, the 12-nation European Community and members of the Conference on Security and Cooperation in Europe, may signal that Milosevic’s ouster will be the price exacted by the West for acceptance of carving up Bosnia at the Muslims’ expense.

The official diplomatic message, leaked to journalists after Milosevic failed to answer it, further isolates the Serbian strongman, who has already been denounced by federal authorities and a growing circle of Serbian intellectuals and opposition parties.

While the Western reprimand might further undermine Milosevic’s eroding prestige, he still commands tens of thousands of heavily armed republic police and paramilitary forces, giving him the power to provoke devastating new conflicts and take much of the nation down with him if opponents move to oust him from power.

Times staff writer Norman Kempster, in Washington, contributed to this report.