The move is part of new joint efforts to improve relations between Croatia's government and the country's Serb minority, 25 years after the conflict that tore the communities apart.

Earlier this month, a Serb official for the first time joined Croatia's annual commemoration of the military victory that crushed the Serb rebellion in 1995, ending the war sparked by Zagreb's declaration of independence from Yugoslavia.

The killings in Grubori, where six elderly civilians were murdered, took place weeks after that offensive on August 25, 1995.

"This is an honour we owe for what happened 25 years ago, which caused moral horror for me personally," said Croatian President Zoran Milanovic, joined by two deputy ministers at the gathering where roses were laid in front of a large wooden cross.

"Serbs and Croats are the most similar nations in Europe," he added.

Deputy prime minister Tomo Medved, a war veteran, said it was "necessary to make efforts in building a stronger community for the sake of all Croatian citizens".

"Croatia, as a victor mourns all those who were killed, especially civilians, and it is our duty to honour the victims," he added.

The war crime in Grubori was carried about by a special Croatian anti-terrorist unit, a court found, but no individual perpetrators have been convicted due to a lack of evidence.

Many Serbs still decry the trial in Croatia that acquitted two officers, while a third committed suicide.

Milorad Pupovac, president of a Serb political party in Croatia, lamented the court's failure "to establish who committed the crime".

The victims "were killed while taking care of their cattle, shelling peas or sitting in a wheelchair. Murdered with their arms up," he said at the ceremony.

More than 20,000 people were killed in the conflict, while some 200,000 Serbs also fled Croatia after the end of the war.

Around half have since returned and today ethnic Serbs make up some 4.5 percent of the country's 4.2 million population.