The UN Special Rapporteur on Torture says there is no excuse for the brutal treatment of tied-up detainees as seen in last week's video of a prolonged beating in Fiji.
Juan Mendez says he is in communication with the Fiji government and asking relevant questions over the matter.
Fiji police say they are investigating the incident but the Prime Minister, Commodore Frank Bainimarama, has not condemned the violence.
Professor Mendez says some governments argue that the conduct of detainees sometimes necessitates the use of force against them.
He says international law clearly prohibits this.
"Nothing justifies the treatment. Not even the worst offender should be treated in any way that causes pain and suffering equivalent to torture or cruel, inhuman and degrading treatment. The prohibition is absolute, so there is no excuse for it and much less any excuse by the previous conduct of the victim."