The Hague - 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.
"Time and again, new governments have come into power in Serbia who are unwilling to confront the truth about the events that began more than two decades ago," Crnic-Grotic told judges.
"Croatia's only resort to justice appears to be this court," she said.
Croatia in 1999 dragged Serbia to the ICJ on genocide charges relating to the 1991-95 war following the collapse of the former Yugoslavia.
Belgrade, disappointed by the disintegration of Yugoslavia at the time, went on the offensive to defend what it said was a threat to Serbian ethnicity.
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" in the resulting war.
Some 20 000 people died in the conflict, one of several to shake the Balkans in the 1990s.
Judges on Monday watched a video of the remains of the city of Vukovar after it fell to Serb-backed forces in November 1991, after a harrowing three-month siege during one of the war's darkest episodes.
Zagreb now 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".
But 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% 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 for an agreement on a withdrawal.
Serbia's representative Sasha Obradovic on Monday dismissed Croatia's claim as nothing new.
"What we heard [today] is a recital of what we heard 10 years ago," he told journalists after the opening day's hearings.
"It's a little sad that we did not hear anything about the destiny of the Serbian people in... Croatia among whom there were also many victims," he said.
"Of course, horrific crimes were committed in Croatia and it should not be forgotten, but those crimes didn't fulfil the legal requirements of the crime of genocide," he said.
Serbia is to present its arguments on 10 March.