A frail elderly couple who were once among the most feared ministers of the murderous dictator Pol Pot were arrested today in Cambodia.

Ieng Sary and his wife Ieng Thirith, who were Pol Pot's brother-in-law and sister-in-law, were taken into custody at dawn in a raid by armed police on their villa in Phnom Penh.

The couple were transported in a police convoy to Cambodia's UN-backed genocide court in the city's western suburbs, where they were due to appear later today charged with crimes against humanity, and in Ieng Sary's case with war crimes.

The arrests brings the number of Pol Pot's inner circle now in custody in Cambodia to four, after the dictator's deputy, Nuon Chea, and Duch, who ran the infamous Tuol Seng torture centre, were arrested earlier this year.

The trials are expected in 2008, although tension and decade-long delays in setting up the so-called "Killing Fields" court have led some to fear that the elderly defendants could die before being brought to justice.

The Khmer Rouge caused the deaths of an estimated 1.7 million of its fellow countrymen and women during its four year reign of terror from 1975-1979. The true death toll will never be known.

With the backing of China, Pol Pot and his ministers abolished religion, schools and money in the hopes of creating a Communist rural utopia. Anyone suspected of failing to support the regime was exiled to vast farms, where many died of starvation and overwork.

The worst violence was reserved for those deemed to be intellectuals, who were were purged and slaughtered wholesale.

As Pol Pot's foreign minister and deputy prime minister, Ieng Sary was the public face of the secretive Khmer Rouge regime.

He has been accused of involvement in the killings, persuading hundreds of educated Cambodians, who had fled the Marxist revolution, to return to their country - only to be tortured and executed. Some of those who died were diplomats in his own ministry.

"Ieng Sary appears to have contributed to the perpetuation of atrocities... by encouraging the Party's execution policies," write Stephen Heder and Brian Tittemore in one of the definitive studies of the era.

Ieng Thirith, his wife, was the regime's social affairs minister and has been accused of helping to plan its policy of mass killings.

Both continued to defend the regime to the wider world for years after it was overthrown and became a guerilla movement in 1979. Ieng Sary was tried in his absence the same year and found guilty of genocide.

The couple have lived in retirement since surrendering and making a deal with the authorities in 1996. Their defection hastened the collapse of the Khmer Rouge. Pol Pot himself died in 1998 in one of his remote strongholds.

As part of their deal Ieng Sary was granted a royal pardon, but legal experts say that it is however still possible for him to face other charges.

Court investigators are said to be hunting a fifth suspect, believed to be Khieu Samphan, 76, who served as head of state.

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