Serbs Launch Attack on Muslims in Northeast Bosnia : Balkans: Offensive reported near Brcko is the first significant fighting since a weekend cease-fire.

Serbs launched a major attack Thursday on Muslims clinging to the outskirts of a key city in northeastern Bosnia, seeking to clear more territory for a Greater Serbia, government officials said.

Bosnia’s Muslim president, Alija Izetbegovic, spoke of a “massive offensive” against Muslim forces around Brcko on the Croatian border. Sarajevo radio said Muslim villages around the city were entirely in flames.

There was no independent confirmation of the Serb attack. But if the reports were accurate, it would be the first significant Serb-Muslim fighting since the two factions signed a cease-fire Sunday.

In southwestern Bosnia, meanwhile, sporadic shooting continued in Mostar between Muslims and Croats despite a truce meant to halt four days of pitched battles between the nominal anti-Serb allies.

The European Community warned neighboring Croatia that it could be hit with international sanctions if Bosnian Croats did not immediately halt attacks on Muslims. It cited the U.N. trade embargo imposed on Serb-dominated Yugoslavia for backing the Bosnian Serb rebellion.

Predominantly Muslim Brcko fell early to Serb forces in the 13-month-old war that has left 150,000 people dead or missing. But outgunned Muslim-led government troops have managed to keep a tenuous presence in some outlying sections of the city, threatening the Serbs’ corridor to Serbia itself.

Sarajevo Radio said the Serb offensive was a matter of “life or death of over 50,000” Muslims around Brcko.

Jovan Divjak, deputy commander of Bosnian government troops, said the Serbs were using 50 tanks in the Brcko offensive.

U.N. officials in Sarajevo complained that the Bosnian Serb military commander, Gen. Ratko Mladic, was blocking aid convoys to eastern Muslim enclaves because of a dispute over a demilitarization agreement.

“They think demilitarization of an area means collecting the weapons of the other side and not withdrawing” besieging Serb troops, said Col. Marcel Valentin, acting U.N. commander in Sarajevo.

In embattled Mostar, fighting quieted, but tensions remained high. U.N. officials said Croat and Muslim troops refused to follow an agreement by their leaders for the combatants to return to their barracks by midday.

Spanish peacekeepers who were forced to leave Mostar on Sunday because of heavy fighting returned Wednesday night despite continuing combat, said Cmdr. Barry Frewer, the U.N. spokesman in Sarajevo.

The cease-fire agreement called for the unconditional release of civilian detainees by the end of the day. Some had been released by evening, U.N. spokesman Peter Osborne said in the Croatian capital, Zagreb.

Muslims and Croats have fought together against Serbs during most of the war, but the recent clashes in Mostar have undermined their shaky alliance.

Croatian militia spokesman Veso Vegar said more than 40 Croat soldiers and civilians were killed in Mostar since Sunday. There were no casualty figures from Muslim commanders.