Serbia's prime minister on Saturday condemned the "monstrous crime" of Srebrenica, ahead of attending a ceremony marking the 20th anniversary of the massacre committed by Bosnian Serb forces.

"There are no words to express regret and pain for the victims as well as anger and bitterness towards those who committed that monstrous crime," Aleksandar Vucic said in a public letter published shortly before he was to attend a commemoration ceremony in the eastern Bosnian town.

"Serbia clearly and unambiguously condemns this horrible crime and is disgusted with all those who took part in it and will continue to bring them to justice," he said.