Not Party to New Cease-Fire, Croats Clash With Bosnia Muslims : Balkans: Truce seems to ease pressure for Western intervention, but both sides in a broken alliance fight on in Mostar.

A new cease-fire came into force in Bosnia on Sunday, but fierce clashes between Croats and Muslims overshadowed the latest attempt to halt Europe’s bloodiest conflict since World War II.

The truce, which appeared to ease pressure for Western military intervention, took effect at noon under an agreement struck between the commanders of Muslim-led government forces and the Bosnian Serb army.

But the deal did not include Bosnia’s Croats, estranged allies of the Muslims in the year-old war.

Both sides in the broken alliance said their forces were fighting each other on the streets of Mostar, a Bosnian Croat stronghold in the southwest, and both reported civilian casualties.

Fires broke out in Mostar amid fierce fighting, and hundreds of civilians left the city under Croatian military guard, U.N. peacekeeping officials said in the Croatian capital, Zagreb.

Cmdr. Barry Frewer, chief spokesman for the U.N. Protection Force in Bosnia, said U.N. military observers in Mostar along with officers from a battalion of Spanish soldiers reported that Croatian forces launched an attack on the Muslims in the city starting about 5 a.m. and that shells exploded throughout the town all day. The headquarters of the Bosnian army and several other buildings were on fire, Frewer said.

Croatian forces blocked the Spanish troops from entering the city, so they set up an observation post on its outskirts. There, Frewer said, the Spaniards observed “orderly lines of women and children being led into a soccer stadium by Croat paramilitary groups.”

Men were being separated from the women and children and taken to another compound, he said. It was unknown how many people were being rounded up, he added.

The battles in Mostar appeared to mark another escalation in a Croat-Muslim fight for territorial spoils unleashed by a U.N.-backed peace plan that would carve up Bosnia along ethnic lines.

A string of truces has so far failed to halt the carnage. The latest was agreed to by Bosnian Serb army Gen. Ratko Mladic and Muslim commander Sefer Halilovic during marathon talks at Sarajevo airport under U.N. mediation.

Mladic and Halilovic also struck a separate agreement declaring the Muslim enclaves of Zepa and Srebrenica in eastern Bosnia to be “demilitarized zones” and calling for the withdrawal of weapons and combatants.

U.N. military observers finally reached Zepa on Sunday, a U.N. spokesman said.

The enclave, where at least 30,000 Muslims are believed to be trapped by a siege under way since March last year, had been under fierce assault by Serbian forces since Tuesday, according to Bosnia’s Muslim-led government.

The observers, who had been trying to enter the town since Wednesday, reported that they had found 10 people dead in a mosque and two people badly wounded in a cellar, Frewer said.

The observers said the cease-fire appeared to be in effect, as there was no fighting. But Frewer stressed that their report contained only their first impressions of the situation in Zepa.

Meanwhile, sporadic small arms fire crackled across the Bosnian capital minutes after the truce clock began ticking.

“If we are to judge by the three hours that have expired, we can’t be optimists,” Muslim President Alija Izetbegovic said, referring to sniping and some shelling in Sarajevo and what he said was sporadic shooting elsewhere.

But Gen. Philippe Morillon, commander of U.N. forces in Bosnia, said the deal needed time to work, and British Defense Secretary Malcolm Rifkind said the West wanted to see if it would stick before considering air strikes.

Bosnian Serb leader Radovan Karadzic said he wanted President Clinton and other world leaders to meet a Serbian delegation that would explain why the self-styled Bosnian Serb parliament rejected the Vance-Owen peace plan.

Karadzic warned of “catastrophic developments” if the West made decisions about Bosnia on the basis of what he called incomplete and distorted information on the war.

“Deeply convinced that you will understand our reasons (for rejection), we are asking you to receive a Serbian delegation which would inform you about the essence of our understanding of the crisis and our views of the peaceful ways to overcome it,” Karadzic said in a letter.

The Bosnian Serb news agency SRNA said Karadzic had sent the letter to Clinton, Russian President Boris N. Yeltsin and the leaders of Britain, France and Germany.

Mladic, whose forces control 70% of Bosnia, said Croat-Muslim fighting proved that the U.N. peace plan could not work.

“The plan is a strategic and political monstrosity and has as much chance of surviving as Siamese twins,” he told the Yugoslav news agency Tanjug.