The UN human rights chief has urged Senegal to reconsider sending former Chadian President Hissene Habre back to Chad, where he has been sentenced to death in absentia.
Navi Pillay expressed concern that Mr Habre could be tortured in Chad.
On Friday, Senegal said Mr Habre would be flown to Chad on 11 July.
Mr Habre is blamed for killing and torturing tens of thousands of opponents between 1982 and 1990, charges he denies.
"I urge the government of Senegal to review its decision," Ms Pillay, the UN High Commissioner for Human Rights, said in a statement on Sunday.
"As a party to the Convention Against Torture, Senegal may not extradite a person to a state where there are substantial grounds for believing he would be in danger of being subjected to torture."
Human rights groups have also expressed concern over Senegal's decision, saying Mr Habre would not get a fair trial in Chad.
He has been living in the Senegalese capital, Dakar, since he was ousted in 1990. A 1992 Truth Commission in Chad accused him of being responsible for widespread torture and the death of 40,000 people during his eight-year rule.
In 2008, he was sentenced to death in absentia for planning to overthrow Chad's government.
Senegal arrested Mr Habre in 2005, after he was charged by Belgium with crimes against humanity and torture.
Belgium has also been asking the International Court of Justice to have Mr Habre extradited to Belgium, where survivors have filed a case against him.