Croatia on Monday said Serbia was "in denial" over genocide allegedly committed in the early 1990s, at the start of a court case that threatens to further sour relations between the Balkan neighbours.
"Many political leaders in Serbia have maintained an attitude of denial" about the alleged genocide, Croatia's representative Vesna Crnic-Grotic told the Hague-based International Court of Justice (ICJ).
"It is to our great regret that this (attitude) persists today," she told the UN's highest court, where the case finally opened on Monday.
Croatia in 1999 dragged Serbia to the ICJ on genocide charges relating to Zagreb's 1991-95 war of independence following the collapse of the former Yugoslavia.
Croatia accused Serbia of "ethnic cleansing" which it said was "a form of genocide which resulted in large numbers of Croatian citizens being displaced, killed, tortured or illegally detained, as well as extensive property destruction".
Some 20,000 people died in the war, one of several to shake the Balkans in the 1990s.
Zagreb wants judges to order Belgrade to pay compensation for damage "to persons and properties as well as to the Croatian economy and environment... a sum to be determined by the court".
Belgrade responded with a countersuit in 2010, saying some 200,000 ethnic Serbs were forced to flee in 1995 when Zagreb launched a military operation to retake its territory.
Following that counter-offensive, called Operation Storm, ethnic Serb numbers in the area dwindled from 12 percent to four percent.
Belgrade was also outraged in 2012 when Operation Storm's Croatian military commander, Ante Gotovina, was acquitted on appeal by the Hague-based International Criminal Tribunal for the former Yugoslavia (ICTY).
So far the ICJ, which rules in disputes between states, has recognised only one genocide case since opening its doors in 1946.
In 2007 it ruled that genocide took place in 1995, at Srebrenica in neighbouring Bosnia, when almost 8,000 Muslim men and boys were slaughtered by Bosnian Serb troops after they overran a UN-protected enclave.
Genocide is the most serious of international crimes but also one of the hardest to prove.
Both Zagreb and Belgrade said previously they would consider withdrawing their cases before the ICJ if certain conditions were met.
But politicians in both Balkan neighbours in recent weeks said it was too late too late for an agreement on a withdrawal.
Serbia is to present its arguments on March 10.