On Thursday, Obama absolved CIA officers from prosecution for harsh, painful interrogation of terror suspects under the former Bush administration.
The announcement was met with disappointment from human rights groups and former detainees who condemned such methods as torture.
In a brief telephone interview with The Associated Press, Manfred Nowak, an Austrian who serves as a U.N. special rapporteur in Geneva, said the United States had committed itself under the U.N. Convention against Torture to make torture a crime and to prosecute those suspected of engaging in it.
"They are party to the convention and the convention is very, very clear," Nowak said when asked to confirm comments contained in an interview he gave Austria's Der Standard newspaper. "The fact that you carried out an order doesn't relieve you of your responsibility," he said, adding it could be a mitigating factor.
Nowak, who said he would soon travel to Washington for meetings with officials, also called for a comprehensive independent investigation into the matter and added it was important to compensate the victims.
"Now we need to know all the facts - not just bits and pieces," Nowak said. "First you need the truth, and then you need justice."
The Obama administration on Thursday also released secret CIA memos detailing interrogation tactics sanctioned under Bush.
The memos authorized keeping detainees naked, in painful standing positions and in cold cells for long periods of time. Other techniques included depriving them of solid food and slapping them. Sleep deprivation, prolonged shackling and threats to a detainee's family also were used.
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By Associated Press Writer Veronika Oleksyn