PALE, Bosnia -- U.S. mediator Jimmy Carter said Monday that Bosnian Serb leaders had agreed to an immediate four-month ceasefire in their war with Moslems and were willing to discuss a Big Power peace plan.


The former U.S. president said after several hours of talks with Bosnian Serb leader Radovan Karadzic he would return to Sarajevo to put the ceasefire proposal to the Moslem-led Bosnian government.


Carter told a news conference, "The Bosnian Serb side agreed to an immediate ceasefire and to negotiate a lasting cessation of hostilities.


"While a ceasefire is in place, Bosnian Serb leaders agree to discuss peace on the basis of the (Big Power) 'contact group' plan at a mutually acceptable site."


Carter was expected to return to Sarajevo to resume contact with the Bosnian government Monday evening or Tuesday morning.


The former U.S. president, conducting a personal peace initiative, said he had informed the White House.


The agreement was signed by Carter, Karadzic and Bosnian Serb Army commander General Ratko Mladic, and promised to continue lifting Serb harassment of UN aid operations.


Karadzic has been resisting the peace plan since the summer, claiming it would make his self-proclaimed Bosnian Serb republic economically and militarily unviable.


The plan would divide Bosnia, giving 51 percent to a federation of Moslems and Croats, and the rest to Serbs, who today control 70 percent.


The Serbs have been pressing for a permanent ceasefire for months, claiming they have won the war, and they appeared to have made a concession by agreeing to four months.


The Bosnian government has so far been prepared to sign only a three-month ceasefire, reserving itself the right to resume fighting to try to regain territory if talks fail. Carter caused controversy at the start of his meeting when he said Bosnian Serbs wanted peace and were misunderstood by the American people.


He said, in front of television cameras, "It may be that today is one of the rare chances to let the world know the truth, and to explain the commitment of the Serbs to a peace agreement."


Bosnian Serbs, widely blamed as the aggressors in the war, have rarely heard such endorsement from international negotiators.


Karadzic told Carter, "Many American believe we came from Serbia and invaded someone's territory. If that were the case, we would have been the aggressor. This is not the case. We are natives here. Here are our roots."


Carter replied, "I can't dispute your statement that (the American people) have primarily heard one side of the story."


His statement was immediately disavowed by the White House, where spokeswoman Dee Dee Myers said Serbs were the aggressors in the war, and that Americans were fully informed.


Bosnian Serbs made up a third of the pre-war population in Bosnia; Moslems, 44 percent; and Croats, 17 percent. Most of the 200,000 dead in the fighting have been Moslems.


Carter arrived in Pale from Sarajevo, where he had talks with Bosnian President Alija Izetbegovic on Sunday. He also met Bosnian Prime Minister Haris Silajdzic and Croatian President Franjo Tudjman in Zagreb.


Carter crossed into Serb-held territory from the Moslem sector of Sarajevo after apparently satisfying himself that Karadzic had honored promises to end a blockade of Sarajevo airport and UN aid convoys to Moslem civilians.


Carter wanted to travel to Belgrade on Tuesday, probably to try to meet Serbian President Slobodan Milosevic.