By Agence France-Presse
A UN-backed court in Cambodia dismissed an appeal against lifetime jail sentences meted out to Nuon Chea, 90, Khieu Samphan, 85.
The pair were senior leaders of a regime responsible for the deaths of up to two million Cambodians from 1975-1979.
"The long arm of international justice ultimately can prevail," David Scheffer, the UN Secretary-General's envoy to the tribunal, told reporters after the verdict.
"Holding senior leaders accountable for the perpetration of atrocity crimes under their leadership, does happen, it does ultimately occur," he added.
He then mentioned a number of specific countries where leaders should "take note that what happened today".
The were the Philippines, South Sudan, Sudan, the Central African Republic, Syria and North Korea. He also named the Islamic State group, which has committed widespread atrocities across swathes of Iraq and Syria.
"What happened today in this courtroom ultimately can reach their domain because international justice is not backing down," he added.
The Khmer Rouge regime dismantled modern society in Cambodia in their quest for an agrarian Marxist utopia, killing vast numbers and leaving a generational scar.
Some analysts have compared the Khmer Rouge to the Islamic State group, for their ruthless pursuit of revolution and sheer barbarity.
But while the court has brought a handful of senior Khmer Rouge leaders to book, the vast majority of perpetrators remain unpunished.
The movement's leader Pol Pot died in 1998, and the government of Cambodian strongman Hun Sen has become increasingly wary about prosecuting lower level cadres.