Bosnian Serbs, already under pressure for rejecting a Big Power peace plan, faced further isolation on Monday after a U.N. war crimes tribunal named two of their leaders as suspected war criminals.

The U.N. tribunal named Bosnian Serb leader Radovan Karadzic, his military commander General Ratko Mladic and his former secret police chief as war crimes suspects in former Yugoslavia.

"My office is currently investigating the question of responsibility of these prominent individuals for genocide, murder, rape, torture and the forced removal of many thousands of civilians from large parts of Bosnia and Herzegovina..," chief prosecutor Richard Goldstone told reporters in The Hague.

He said the investigations could lead to formal charges by the end of the year.

It was the first sign that the U.N. tribunal planned to target prominent figures who have been condemned by the West for their role in the three-year old conflict. The tribunal has so far indicted 22 people, all Serbs, but none in promiment, public positions.

If indictments are issued, the Bosnian Serb leaders would face arrest if they travelled abroad, as Karadzic has done for peace talks in Geneva.

Some U.N. officials in Sarajevo privately expressed concern about the timing of the announcement, saying it could provoke more retaliation by angry Bosnian Serb forces laying siege to Sarajevo.

Officials said the tribunal's move could damage already strained relations between the Serbs and the U.N. at a time when the peacekeeping mission is struggling to broker an extension of the current ceasefire in Bosnia, which expires in six days.

Besieging Serb forces in recent weeks have forced the suspension of relief flights into Sarajevo and refused to allow U.S. and German diplomats to enter the city on Friday.

Karadzic had defended blocking the Western envoys, saying Bosnian Serbs were angry at a recent U.N. Security Council resolution.

The Council, reaffirming the isolation of the Bosnian Serbs, called for tighter enforcement of Serbian President Slobodan Milosevic's military blockade on the Bosnian Serbs.

Milosevic, anxious to win an end to a U.N. trade embargo, broke with the Bosnian Serb political leadership last August over their refusal to endorse a peace plan drawn up by the five-nation Contact Group of the United States, Russia, Britain, France and Germany.

The implications of The Hague tribunal's announcement for Milosevic's political rift with Karadzic remained unclear.

But one Bosnian Serb source said: "this indicates a complete failure of Milosevic's policy."

The source dismissed the tribunal's statements which he said were the result of the United States exerting pressure on the United Nations to punish the Serbs.

The Bosnian Serbs hold about 70 percent of the former Yugoslav republic after three years of bloody war and have consistently refused to back the Contact Group peace plan, which would divide the country roughly in half between the Serbs and a Moslem-Croat alliance.