A handshake between Kosovan prime minister Hashim Thaci and former Serbian president Boris Tadic has triggered outrage in both countries.
The two attended a summit of current and former regional leaders and European Union officials in Dubrovnik, Croatia.
It was the first time the two had met face-to-face since the 1998-99 war between Serb government forces and independence-seeking Kosovo Albanians that left thousands dead and millions homeless.
Mr Thaci, a former ethnic Albanian guerilla commander, and Mr Tadic, who lost a May presidential election against nationalist opponent Tomislav Nikolic, shook hands during the summit, but apparently held no official talks.
Serbia, backed by Russia, has refused to recognise the independence of its former province declared in 2008, and has boycotted all international meetings attended by high-ranking Kosovo officials. Mr Thaci has been officially branded a "terrorist" in Serbia.
Serbia's prime minister designate, Ivica Dacic, said Mr Tadic's presence at the summit is an "enigma", and asked the former pro-Western Serbian leader to clarify his policies now that he is an opposition official.
"It seems Tadic has led one policy as president and a different one as an opposition leader," said Mr Dacic, who has ditched his alliance with Mr Tadic from the previous government and is now trying to form a new Cabinet with nationalists. "Concerning the handshake with premier Thaci, there is nothing historic about it," Serbia's state Tanjug news agency quoted Mr Tadic as saying. "But it's good that the representatives of (ethnic) Albanian and Serbian nations are meeting. We have a problem between us that we need to solve."
In Kosovo, the handshake sparked condemnation among opposition ethnic Albanian politicians who said it damages Kosovo's future.
"This shows the current government is ready to meet the same Serb politicians that are the main reason Kosovo remains (ethnically) divided," said Glauk Konjfuca, of the opposition Self-determination party, which is opposed to any talks with Serbia. "This means Thaci is ready to meet Nikolic and Dacic," he added.
Arben Gashi, of the main opposition party, said the meeting breached Kosovo's constitution that says a Kosovo official can only meet counterparts that recognise Kosovo as an independent state.