Verdict a signal to other perpetrators

The US has welcomed Cambodia's landmark genocide verdict and said it served as a warning that perpetrators of mass atrocities, "even those at the highest levels", will eventually face justice for their crimes.

A war crimes tribunal in Cambodia found the Khmer Rouge's former head of state Khieu Samphan, 87, and "Brother Number Two" Nuon Chea, 92, guilty of genocide on Friday and sentenced them to life in prison.

The historic verdict comes nearly 40 years after the Khmer Rouge were expelled from Cambodia following a four-year reign of terror that left about a quarter of the population dead from starvation, mass executions, and overwork.

"Their crimes were numerous, calculated, and grave," US State Department spokesperson Heather Nauert said, commending the courage of the victims and witnesses who testified during the trial.

"Let this be a message to other perpetrators of mass atrocities, even those at the highest levels, including former heads of state, that such actions will not be tolerated and they will ultimately be brought to justice," she said in a statement.

Cambodia's neighbour Myanmar has come under fire in recent months for its handling of the Rohingya crisis, which United Nations investigators believe amounts to "genocide" given the atrocities perpetrated on the stateless Muslim minority.