Extraordinary Chambers in the Courts of Cambodia, Nhet Sok Heng, Associated Press
In this photo released by the Extraordinary Chambers in the Courts of Cambodia, Kaing Guek Eav, center, also known as Duch, who ran the notorious Toul Sleng prison, where up to 16,000 people were tortured before being killed, appears during testimony at the U.N.-backed war crimes tribunal in Phnom Penh, Cambodia, Monday, March 19, 2012. The Khmer Rouge's chief jailer is scheduled to testify against three of the regime's surviving leaders who are accused of crimes against humanity.

PHNOM PENH, Cambodia — A Swiss judge has resigned from the U.N.-backed tribunal prosecuting Khmer Rouge war crimes, a move Amnesty International blames on the Cambodian government's interference with the court's efforts to seek justice for victims of the 1970s atrocities.

Judge Laurent Kasper-Ansermet said Monday that he will step down in May because a Cambodian counterpart has opposed his investigations of new suspects. He said the conflict has created "a dysfunctional situation" on the court, which is seeking justice for an estimated 1.7 million people who died of starvation, exhaustion, lack of medical care or torture during the communist regime's rule.

In a statement Tuesday, Amnesty International called the latest resignation "a significant setback."

The former regime's chief jailer is in prison and three of its leaders are on trial, but Cambodia's leadership opposes extending prosecutions to more Khmer Rouge figures, some of whom have become political allies. The country's powerful ruler, Prime Minister Hun Sen, has publicly chided and threatened the tribunal several times.

"The victims of the Khmer Rouge atrocities must be feeling utter despair," said Rupert Abbott, Amnesty International's Cambodia researcher. "The U.N. must demand that the Cambodian government desists from this political interference, and make clear the consequences should it continue."

Kasper-Ansermet had replaced German judge Siegfried Blunk, who left in October, also citing government interference. Human Rights Watch, though, had accused Blunk of failing to conduct genuine and impartial research beyond the one suspect convicted last year and the top Khmer Rouge leaders currently on trial in the second case to go before the court.

A statement issued by the tribunal said Kasper-Ansermet would resign May 4.

Kasper-Ansermet said a Cambodian colleague, You Bunleng, had constantly contested his authority. He accused Bunleng of "active opposition" to new cases and said that during an informal meeting, Bunleng had refused to even discuss them.

Bunleng could not immediately be reached for comment.

Prosecutors have compiled substantial evidence for so-called Cases 003 and 004, which include two top military commanders who also were leaders in Cambodia's post-Khmer Rouge military, according to confidential court documents obtained by AP. The documents allege both took part in purges that resulted in tens of thousands of deaths.

The government, however, has openly stonewalled. Hun Sen told Ban last year that new cases would "not be allowed." He has warned that new cases could spark renewed civil war, though his opposition likely stems from the many Khmer Rouge officials, like himself, who are now in government and who fear investigators could find new evidence of war crimes.

Earlier Monday, the Khmer Rouge's chief jailer started a week of testimony against three of the regime's surviving leaders who are accused of crimes against humanity.

It was Kaing Guek Eav's first courtroom appearance since February, when the tribunal's appeals court sentenced him to life imprisonment for committing "shocking and heinous" crimes against the Cambodian people.

Duch's testimony Monday covered preliminary information and background about his work as a prison commander but did not address the three senior Khmer Rouge figures being tried.

They are 85-year-old Nuon Chea, the Khmer Rouge's chief ideologist and No. 2 leader; 80-year-old Khieu Samphan, an ex-head of state — both of whom were present in the courtroom. Ieng Sary, the 86-year-old former foreign minister, was granted permission to watch from a private room due to back trouble that prevents him from sitting for long periods.

All three are accused of crimes against humanity, genocide, religious persecution, homicide and torture.

Unlike Duch, who admitted his role and asked for forgiveness, the others insist they did nothing wrong and say they worked on behalf of Cambodians. An estimated 1.7 million people were killed during the regime's rule by execution and torture or died from starvation, hard labor or lack of medical care.

Associated Press writer Todd Pitman contributed to this report from Bangkok.