The conviction of two former Khmer Rouge leaders brought a measure of relief to victims of the Cambodian regime's genocidal rule in the 1970s, but underscored the slowness of a war-crimes tribunal tasked with punishing decades-old atrocities.

Nuon Chea, 88 years old, the Khmer Rouge's chief ideologue and former deputy to late leader Pol Pot, and 83-year-old Khieu Samphan, the former head of state, were sentenced to life imprisonment on Thursday after being found guilty of crimes against humanity—directing murder, political persecution and other inhumane acts related to the mass eviction of city-dwellers and executions of enemy soldiers.

The verdict, coming nearly three years after the trial began, marked the first convictions secured against top-tier regime officials by the United Nations-backed tribunal, long plagued by funding shortfalls and perceived political interference.

Survivors of the Khmer Rouge era hailed the outcome as a historic moment for Cambodian criminal justice, assuaging fears that the aging defendants would die before facing some punishment for their roles in overseeing the deaths of at least 1.7 million people between 1975 and 1979.

But the verdict covers only a small portion of the Khmer Rouge's crimes, and observers say the tribunal must accelerate further proceedings against Messrs. Nuon Chea and Khieu Samphan—starting with a second trial this year that would deal for the first time with charges of genocide, considered the Khmer Rouge's worst atrocity.