SARAJEVO, Bosnia-Herzegovina (AP) _ U.N. troops today tried to find thousands of civilians missing in two Bosnian war zones: Zepa in the extreme east and Mostar in the southwest.

Ukrainian peacekeepers in Zepa, a Muslim enclave, fanned out this morning in search of residents who took to the hills to escape a fierce attack last week by Bosnian Serbs.

Spokesman Ron Redmond for the U.N. High Commissioner for Refugees in Geneva, said a trickle of people had begun to return to Zepa from the hills, where they had camped out in tents or in the open after fleeing Bosnian Serb artillery.

''They're in very, very bad condition,'' Redmond said.

U.N. spokesman Pepe Gallegos said in Sarajevo that the Ukrainians had found many people wearing clothes made from parachutes. Food and medicine were reported in short supply.

U.N. officials also expressed concern about Muslim civilians said to be forcibly taken from their homes by Croatian forces in fighting Sunday in Mostar.

Little fighting was reported in either city today as peacekeepers worked to cement shaky truces.

Bosnia Serbs are attempting to take control of the Zepa region, and Croats in the south have begun an offensive to take land around Mostar. Both regions had been dominated by Muslims.

A meeting of Bosnian army and Croat commanders was scheduled to discuss the Mostar fighting.

About 40,000 people were in the Zepa area, including 6,000 in the town itself, before they fled the Serbs' attack. The first U.N. military observers who reached Zepa on Sunday found only about 50 people - and corpses, including 10 bodies in a mosque.

Zepa, which came under heavy attack last week, was one of six ''safe areas'' declared by the United Nations last Thursday after Bosnian Serbs again rejected an international peace plan.

U.N. officials said they were concerned about the Muslim civilians reportedly cleared out of their homes in Mostar.

''There have been a number of civilians taken from their homes,'' said U.S. Army Capt. Bill Caraktin, a U.N. spokesman in the central Bosnian town of Kiseljak. ''We assume they are being detained or being taken out of the area against their will.''

Caraktin said Mostar seemed quieter.

''The word seems to have reached the forces on the ground,'' he said. ''The heaviest fighting seems to have died down. You get isolated small arms fire.''

In the Yugoslav capital of Belgrade, the top U.N. aid official blasted the Croats.

''The official statements by the HVO (Croatian Bosnian army) are the same as those coming from the Bosnian Serb side a few months ago, claiming the necessity to fight Muslims who allegedly want to establish a Muslim fundamentalist state,'' said Jose Maria Mendiluce at a news conference marking his departure.

''This is the beginning of the second wave of ethnic cleansing,'' he said.

Croat leader Mate Boban claims Muslims attacked first.

The U.N. Security Council late Monday demanded that Bosnian Croat forces allow U.N. officials into Mostar. In a statement, the Security Council said it ''strongly condemns this major military offensive launched by Bosnian Croat paramilitary units which is entirely inconsistent with their signature of the peace plan.''

Similar to the Serb dream of a Greater Serbia, many Bosnian Croats envision a Greater Croatia - even though it means taking land from their nominal allies, the Muslims. Its eastern border would be the Neretva River, which runs through Mostar.

The war in Bosnia began a year ago when Bosnian Serbs rebelled against Muslims and Croats, who voted to secede from Serb-dominated Yugoslavia. More than 134,000 people have been declared dead or missing and over a million have been forced from their homes.

Cool to President Clinton's call for military action against the Bosnian Serbs, the European Community has suggested Washington send ground troops to protect places like Zepa.

EC foreign ministers made the request for American troops during a meeting Monday in Brussels, apparently in response to U.S. attempts to drum up European support for a plan to bomb Serb positions and exempt the outgunned forces of the Muslim-led Bosnian government from a U.N. arms embargo.

Clinton has ruled out committing U.S. ground forces to the Bosnian conflict unless there is a peace plan in operation. He also has backed off in recent days from his military plans, acknowledging opposition at home and abroad.

In another alternative to military action, the European ministers offered to send monitors to the border between Serbia and Bosnia to help Serb President Slobodan Milosevic enforce a blockade on Bosnia's Serbs.

France has about 5,000 troops in the former Yugoslavia, Britain has about 2,500.