The presidents of Serbia-Montenegro and Croatia have apologised for the actions of their citizens in the 1991-95 war between the two countries.
The presidents took observers by surprise
The surprise remarks came during the Croatian president's first visit to Belgrade since the conflict, which followed his country's declaration of independence from Yugoslavia.
"As a president of Serbia-Montenegro, I want to apologise for all the evils any citizen of Serbia and Montenegro has committed against any citizen of Croatia," said Serbian President Svetozar Marovic.
His comments prompted Mr Mesic to rewrite a prepared speech to add his own apology.
"In my name, I also apologise to all those who have suffered pain or damage at any time from citizens of Croatia who misused or acted against the law," Mr Mesic said.
The war left around 20,000 people dead and more than a quarter of million displaced from their homes.
BBC correspondent Matthew Price says the remarks came as a surprise to observers and it is not clear if they are official govermnent apologies or more personal, symbolic statements.
Extreme parties in both countries are likely to be opposed to the remarks, believing there is nothing to apologise for, but most politicians will support the move, he added.
The two countries have been stepping up efforts to mend ties since reformist governments came to power in Zagreb and Belgrade in 2000.
Until recently this trip would not have been possible, says Matthew Price, but both Zagreb and Belgrade know they have to improve regional co-ordination if they are to realise their ambitions of joining the European Union and other international institutions.
The international community has also insisted that Croatia allow ethnic Serbs - most of whom fled during the war - to return.
Mr Mesic's visit was expected to be laden with nostalgia. Twelve years ago, he was based in the city as the Croatian member of Yugoslavia's rotating presidency - effectively the country's last president before the multi-ethnic state collapsed.
He stepped down when the war began.
Only 100,000 of Croatia's displaced people have so far returned, and Mr Mesic's spokeswoman said he wanted to resolve the issue.
"Mesic has already said that the return of refugees was key for Croatia not only due to Europe, but also for the country to prove it is a mature democracy," she said.