PHNOM PENH: Lawyers for a former leader of Cambodia's notorious Khmer Rouge regime asked a U.N.-backed court on Monday to overturn a 2018 verdict against him in a genocide trial, arguing it had insufficient reason to conclude he was guilty.

The Extraordinary Chambers in the Courts of Cambodia (ECCC), sentenced former Khmer Rouge-era President Khieu Samphan, 90, to life in prison for genocide against the Vietnamese people and crimes against humanity.

Most of the estimated 1.7 million victims of the 1975-79 ultra-Maoist regime died of starvation, torture, exhaustion or disease in labour camps, or were bludgeoned to death during mass executions.

Guilty verdicts have so far been reached against three former top members of the regime, but several have died while on trial or before indictments were made.

Khieu Samphan's defence team argued that the trial chamber in its verdict had failed to provide sufficient reasoning and violated its own legal framework.

Lawyer Kong Sam Onn told Supreme Court judges present at the hearing that they should dismiss the chamber's ruling.

"The judgement against Khieu Samphan has no legal effect, it should be null and void," the lawyer said, adding Khieu Samphan had been denied a right to a fair trial.

"If judges do not respect their own law, that's the end and that will be a failure."

Co-prosecutor Chea Leang defended the 2018 judgement saying evidence for the conviction was "extensive, diverse and compelling."

"The appellant was one of the key leaders... who committed gruel and barbarous crimes against his own people for his party's own political and ideological goals," Chea Leang said.