A top member of Cambodia's former Khmer Rouge regime on Wednesday said he would not take the stand during his long-awaited trial, marking a setback for a country seeking closure from one of the darkest chapters of the 20th century.

Former Foreign Minister Ieng Sary, one of four former Khmer Rouge cadres charged by the U.N.-backed court with crimes against humanity and war crimes, said he would refuse to speak during his upcoming trial, but gave no reason.

I will not testify, including answer any question put to me, during any trial, Ieng Sary, 86, said in a statement.

Opening statements are due to be heard on November 21 in the trial of the four most senior surviving members of Pol Pot's ultra-Maoist Khmer Rouge -- Ieng Sary, Brother Number Two Nuon Chea, former President Khieu Samphan and former social affairs minister, Ieng Thirith.

Ieng Sary's vow of silence will be a blow to many Cambodians who hoped the testimony of the four defendants would offer some insight into the motives and ideology of one the world's most mysterious and murderous regimes.

An estimated 1.7 million Cambodians died of torture, execution, disease or starvation from 1975-1979 under the Khmer Rouge. Pol Pot, the French-educated architect of a revolution that has become known as the Killing Fields, died in 1998.

Ieng Sary has sought to have his case thrown out for double jeopardy. He was sentenced to death in absentia by a court created by Vietnamese invaders in 1979 and pardoned by Cambodia's then King Norodom Sihanouk in 1996.

The pardon for Ieng Sary came as part of a peace deal after he and his followers broke with the Khmer Rouge.

The court has been mired in controversy since it was formed in 2005, fraught with delays and accusations of political interference by Cambodia's government.

It has handed down just one sentence, a 35-year jail term commuted to 19 years for Kaing Guek Eav, alias Duch, for his role in the deaths of more than 14,000 people at the notorious S-21 torture centre in Phnom Penh. He has appealed.

(Reporting by Prak Chan Thul; Writing by Martin Petty; Editing by Nick Macfie)