The UN special representative for human rights in Cambodia, Thomas Hammarberg, said Kang Kek Leu - better known as Duch - must be fully protected.

Cambodian officials say they have not decided whether to charge Duch or to use him as a witness in the trial of other Khmer Rouge leaders.

Mr Hammarberg, arriving for a two-week visit to Phnom Penh, said: "We will of course ask to see him so that we can be assured he's well treated in preparation for the trial."

Questioning to begin

Judicial officials are expected to start questioning Duch after he was taken into custody on Sunday.

Cambodian police said that charges might follow.

Kang Kek Leu was taken from Battambang, in the west of the country, to a very secure place, protected by interior ministry officials.

The BBC's Caroline Gluck: "Kang Kek Leu went into hiding fearing for his life"

He was tracked down by journalists last month, living in Battambang as an aid worker and born-again Christian.

In an interview with the Hong Kong-based Far Eastern Economic Review, Duch admitted he had been commandant of the torture centre at Tuol Sleng, known as Security Prison S21.

Bludgeoned to death

Signed confessions are exhibited in the former torture centre

During the Khmer Rouge's four years in power, more than 12,000 men, women and children were interrogated and tortured before being killed in the Tuol Sleng centre in Phnom Penh - only seven survived.

Most of the dead were bludgeoned to death to save bullets and were buried in mass graves on the outskirts of Phnom Penh. A number of foreigners were also killed including two Americans, James Clark and Michael Scott Deeds, whose yacht strayed into Cambodian waters in 1978.

They were captured by the Khmer Rouge and, according to Duch, were tortured with electric shocks for more than a month, forcing them to confess to spying simultaneously for the American CIA and Vietnam, the Khmer Rouge's bitter communist rivals.

A signed copy of James Clark's confession is one of the exhibits at the former torture centre turned museum dedicated to the Khmer Rouge's victims.

Naming names

Brother Number One: Pol Pot died last year

In the magazine interview, Duch also named senior Khmer Rouge leaders involved in the genocide. These included the movement's number two, Nuon Chea, who is reported to have ordered all the killings at Tuol Sleng and is now living freely in western Cambodia since defecting to the government in December.

Duch said that he would be willing to stand before a tribunal for his part in the killings, as well as testifying against others. But days after his interviews were published Duch went into hiding fearing for his life.

So far only the Khmer Rouge's military chief Ta Mok, known as "the Butcher", has been arrested for his role in the Cambodian genocide. Pol Pot, the group's one time overall leader, died in a jungle hideout last April.

Several other surviving members of the group's leadership live freely after having agreed terms of defection with the Cambodian Government.