PHNOM PENH - Cambodia's UN-backed court on Friday charged another former Khmer Rouge member with crimes against humanity, weeks after the country's leader warned against prosecuting further suspects.

Mid-ranking regime cadre Ao An, believed to be 79 and better known as Ta An, is alleged to have carried out crimes at an execution site and in two security centres during the regime's brutal rule in the 1970s.

He appeared before tribunal judge Mark Harmon in northwestern Battambang province to hear the charges but was not arrested, court spokesman Lars Olsen told AFP.

"The charged person is presumed innocent until proven guilty through a final judgement," Olsen said.

A formal indictment of An was not expected before the end of this year, he added.

The news will not be welcomed by Cambodia's strongman Prime Minister Hun Sen - himself a former Khmer Rouge cadres - who spoke out earlier this month against further prosecutions, warning they could ignite a civil war.

Days later the court charged Meas Muth, an ex-navy commander, and Im Chaem, a female former district official, with crimes against humanity.

But a Cambodian judge on the tribunal has not signed off on the charges against the latest suspects - including An - reflecting wider unease over the reach of any fresh probes into a regime that massacred up to two million Cambodians in the 1970s.

Cambodian judges outnumber their international counterparts on the tribunal and can vote down a move to formally indict the pair at a later stage.

Critics say Hun Sen is attempting to thwart the trials of lower level cadres having repeatedly vowed that the current case against former top regime leaders would be the last.

So far only three people have been convicted by the hybrid tribunal, which was set up in 2006, including life sentences for the two most senior surviving Khmer Rouge leaders handed down last year.

Led by "Brother Number One" Pol Pot, who died in 1998 without ever facing justice, the Khmer Rouge dismantled modern society in Cambodia in their quest for an agrarian Marxist utopia.

In its historic debut trial, the court in 2010 sentenced former Tuol Sleng prison chief Kaing Guek Eav, alias Duch, to 30 years in prison - later increased on appeal to life - for overseeing the deaths of 15,000 people.

Last August Nuon Chea, 88, known as "Brother Number Two", and former head of state Khieu Samphan, 83, were given life sentences for crimes against humanity - both have appealed.

The pair are currently undergoing a second trial, for genocide, centred around the killing of ethnic Vietnamese and Muslim minorities, forced marriage and rape.