An associate of former Yugoslav president Slobodan Milosevic has said that he tried unsuccessfully to reverse a hardline policy on Kosovo during the 1999 war.
Under cross-examination by Mr Milosevic in The Hague, Zoran Lilic - who preceded him as Yugoslav president - said that he had urged him in a letter to negotiate an end to the conflict during the Nato bombing.
The BBC's Geraldine Coughlan in The Hague says Mr Lilic was one of the few senior political figures who opposed the policy.
Milosevic was not persuaded by Lilic's letter
Mr Lilic also reiterated statements made on Tuesday that Mr Milosevic had nothing to do with the Srebrenica massacre in Bosnia.
Mr Milosevic himself described the 1995 massacre as "truly tragic".
Mr Milosevic is facing more than 60 charges of war crimes, crimes against humanity and genocide, including responsibility for the massacre.
Mr Lilic said he wrote to Mr Milosevic urging an end to a Serb crackdown on ethnic Albanians in Kosovo and negotiations to stop the Nato bombing of Serbia.
"The fate of Serbia is in your hands, the responsibility is historic," he said, recalling the content of the letter.
But he added that he received no response.
Mr Lilic was later thrown out of Mr Milosevic's Serbian Socialist Party.
Our correspondent says that Mr Lilic could provide the prosecution with crucial evidence linking Mr Milosevic to atrocities in Bosnia and Croatia.
So far, however, much of his testimony appears to be a setback for prosecutors.
Mr Lilic said that the accused had made the "greatest efforts to stabilise" the Balkans, by putting pressure on Bosnian Serb leaders to sign up to the Dayton peace accords which ended the conflict.
Mr Lilic told the tribunal on Tuesday that he was certain that Mr Milosevic had not ordered the Srebrenica massacre, which was the worst single atrocity of the Bosnian war.
In other evidence, Mr Lilic did accuse Mr Milosevic of supporting a Serb paramilitary training camp to fight in Bosnia shortly before the war ended.
Mr Lilic was Yugoslav president from 1993 to 1997, although the post then wielded little real power. At the time, Mr Milosevic was Serbian president, but he then took over the Yugoslav presidency and turned it into a more significant power base.
Both men were at the time members of the body in charge of defence strategy, the Supreme Defence Council.
Mr Lilic last year declined to give evidence because he said he was not free to divulge state secrets. He now says he has been given permission by Belgrade to give evidence, but there were limits on what he could say.