A Serbian court has sentenced four former state security members to up to 30 years in prison each for the killing 20 years ago of a prominent journalist who was a fierce critic of then-strongman Slobodan Milosevic.

Slavko Curuvija, the owner and editor of two independent newspapers at the time, was gunned down outside his home on April 11, 1999, just days after the start of NATO's air campaign launched to stop Serbia's crackdown in Kosovo.

The trial for Curuvija's murder was seen as important in dealing with Milosevic-era crimes against regime critics and media freedom.

The court on Friday sentenced the ex-head of state security Rade Markovic and another man to 30 years in prison, and the two others to 20 years in jail each.