The UN Special Rapporteur on Torture Juan Mendez says the Fiji government must not allow any cover-up of torture incidents.
This follows the emergence of a nine-minute video showing the beating of two men said by Fiji police to be recaptured prisoners.
Fiji police say they are investigating but the Prime Minister, Commodore Frank Bainimarama, has not condemned the violence, but instead defended the work of security personnel to protect citizens from hardened criminals.
While Professor Mendez says his communication with the Fiji government over the matter is confidential, its obligation under international law is very clear.
"There has to be an aggressive investigation, and prosecution and punishment of all those who may be responsible. So evidently we expect governments to conduct those investigations impartially and independently, not necessarily taking sides, but also not covering up. Of course that's what we expect to hear from the government of Fiji in the next couple of weeks."