The special rapporteur on torture Manfred Nowak said the accusations were rumours at this stage, but urged the US to co-operate in an investigation.

He said the UN wants lists of the places of detention and those held.

The comments come five days after the UN accused the US of stalling on their requests to visit Guantanamo Bay.

Investigators have been asking to visit the jail in Cuba to carry out checks into allegations of human rights abuse.

The UN said for over a year there had been no response to its requests, and it would begin an inquiry into alleged abuses with or without US co-operation.

Washington had yet to grant their request, Mr Nowak said.

He told the BBC there were a number of allegations from reliable sources that the US was holding terrorist suspects in secret places of detention, including vessels abroad.

He said that according to the reports, the ships were believed to be in the Indian Ocean.

Mr Nowak said the charges of secret detention camps were very serious, amounting to enforced disappearances.