Last Thursday Nowak and three other UN human rights experts said they were opening an inquiry into the US detention camp at Guantanamo Bay, Cuba, where Washington has been holding more than 500 people without trial, and into other such locations.
The United States has neither refused nor granted requests by Nowak's group to visit Guantanamo.
"We have accepted, upon the request of the State Department and
Pentagon, to limit our investigation for now to Guantanamo, but even in accepting this we have not had a positive response" to the request for a visit, Nowak said.
He said that if the "investigation into Guantanamo leads us to other things, we will follow them. We will bring up all these matters to the US government and expect Washington to say officially where these camps are."
The use of prison ships would allow investigators to interrogate people secretly and in international waters out of the reach of US law, British security expert Francis Tusa said.
"This opens the door to very tough interrogations on key prisoners before it even has been revealed that they have been captured," said Tusa, an editor for the British magazine Jane's Intelligence Review.
Nowak said the prison ships would not be "floating Guantanamos" since "they are much smaller, holding less than a dozen detainees."
Tusa said the Americans may also be using their island base of Diego Garcia in the Indian Ocean as a site for prisoners.
Now how come they need all this secrecy? Where's Osama bin Laden?