United Nations appeal judges have significantly expanded the war crimes sentences of two allies of the late Serbian president Slobodan Milosevic.

Jovica Stanisic and Franko Simatovic were held responsible by judges for involvement in crimes across Bosnia and in one town in Croatia as members of a joint criminal plan to drive out non-Serbs from the areas during the Balkan wars in the 1990s.

The appeals chamber at the International Residual Mechanism for Criminal Tribunal in The Hague overturned their acquittals of involvement in the criminal plan and raised the sentences of Stanisic and Simatovic from 12 to 15 years.

Neither man showed any emotion as presiding judge Graciela Gatti Santana passed sentence.

Stanisic was in court for the hearing, while Simatovic watched by video link from a UN detention unit.

Presiding Judge Graciela Gatti Santana said the two men, both now in their 70s, “shared the intent to further the common criminal purpose to forcibly and permanently remove the majority of non-Serbs from large areas of Croatia and Bosnia and Herzegovina through the commission of the crimes charged in the indictment”.

The appeals ruling brings to an end the longest-running war crimes prosecution dating back to the conflict in the early 1990s.

Judge Gatti Santana called the ruling a “milestone” for the court – which deals with cases left over from the now-defunct UN war crimes courts for the former Yugoslavia and Rwanda – as it was the final case dealing with war crimes from the conflict that erupted in the early 1990s as Yugoslavia crumbled.

Stanisic and Simatovic were convicted two years ago of aiding and abetting murder and other crimes committed by Serb paramilitaries in 1992 in the Bosnian town of Bosanski Samac, but were acquitted of responsibility for other crimes.

The appeals chamber reversed both those findings and raised their sentences.

The length of the case underlines the complexity of successfully proving war crimes in international courts, amid international calls for perpetrators of atrocities during the current war in Ukraine to be brought to justice.

Stanisic, a former head of Serbia’s State Security Service, and Simatovic, a senior intelligence operative with the service, are the only Serbian officials to have been convicted by a UN court of involvement in crimes in Bosnia.

Milosevic was put on trial for his alleged involvement in fomenting the bloody conflicts that erupted as Yugoslavia crumbled but he died in his cell in 2006 before verdicts could be reached.

Stanisic and Simatovic were initially acquitted a decade ago by the UN’s Yugoslav war crimes tribunal but an appeals chamber later ordered a retrial.