The US is due to appear before the UN Committee on Torture for the first time since launching its war on terror following the 9/11 attacks.
Thirty senior officials from the departments of state, defence, justice and homeland security will testify in public at the hearing in Geneva.
They are likely to face tough questions about practices used in the US' anti-terror drive, correspondents say.
Rights groups accuse the US of flouting the UN Convention against Torture.
They accuse the US of allowing the torture and inhumane treatment of foreign terror suspects at their detention centres in Afghanistan, Iraq, Guantanamo Bay and elsewhere.
'Huge significance'
This is the first time since 2000 that the US has testified publicly before the committee, which, as a signatory to the UN Convention against Torture, it is required to do.
Ten legal experts will cross-examine the US team, led by state department legal adviser John Bellinger in public hearings that are due to continue until Monday.
The hearings have huge significance, says Jennifer Daskal of Human Rights Watch.
"What makes this so remarkable is that this is the first time the United States is accountable for its record on torture with regard to some of the practices implemented after 9/11," she says.
Names and numbers
The committee will want to know about alleged CIA secret prisons and the prisoners no-one has access to, the BBC's Imogen Foulkes in Geneva says.
According to a UN document, the committee will demand to know the number and nationalities of those being held.
It will also ask for details of those taken abroad to third countries, in a process known as extraordinary rendition.
They may ask about the precise measures the US has taken in the wake of the abuse at Iraq's Abu Ghraib prison to ensure it does not happen again.
More awkward still, they may want to know if there has been an independent inquiry into the possibility that high-ranking government officials authorised torture, our correspondent adds.
The US has insisted it is "unequivocally opposed" to torture and remains committed to the UN ban.
The UN committee does not have formal powers and cannot impose sanctions, but signatories are expected to act on the recommendations it will publish following the hearings.
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