U.N. human rights experts have begun an investigation into U.S. detention facilities for terrorist suspects and allegations that there are secret prisons, one of the project leaders said Wednesday.

Manfred Nowak, the U.N.'s special expert on torture, said some undeclared holding areas could include U.S. Navy ships in international waters. He said there were "serious" allegations to that effect from Amnesty International (search) and other non-governmental human rights groups.

"I have heard these rumors and we have to follow them up," he told The Associated Press, urging Washington to cooperate with the investigation.

U.S. Air Force Lt. Col. John Skinner, a Pentagon spokesman, said the Defense Department "does not operate detainee detention facilities on Navy warships."

The department's "detainee detention facilities are in Iraq, Afghanistan, and Guantanamo," Skinner said.

Nowak, a Vienna law professor, is one of several independent human rights experts appointed by the 53-nation U.N. Human Rights Commission (search), the United Nations' top rights watchdog.

Nowak, who also reports to the U.N. General Assembly, has great autonomy in deciding what to investigate and did not need to seek outside approval in launching the inquiry into U.S. detention practices and locations.

The United States has criticized the commission because its members include countries with poor human rights records. But the experts operate independently and sometimes reproach their own countries for violations.

Jose Diaz, spokesman for Louise Arbour, the U.N. High Commissioner for Human Rights, — the U.N's rights chief — said Nowak had "great authority in investigating suspected abuses."

"Our long-standing position is that we encourage countries to cooperate with the special rapporteurs of the commission," said Diaz. "Their dialogue with the U.S. should continue so that they can be allowed to carry out their work."

Nowak said he and three fellow experts decided last week to launch the inquiry without waiting for assurances of U.S. cooperation after holding off for more than three years in hopes Washington would give members access to Guantanamo Bay (search) and other facilities holding suspected terrorists.

Nowak expressed disappointment at a lack of U.S. response. Still, he said, he was assured after recent high-level meetings with U.S. officials that the request to visit Guantanamo Bay was "being given highest consideration at the top level of the State Department (and) the Pentagon."

He said that the four-member team also would like to visit Abu Ghraib prison in Iraq and other U.S.-run sites known to hold terrorist suspects, as well as to track down the allegations of clandestine prisons — including reports of U.S. Navy ships in extraterritorial waters in the Indian Ocean and elsewhere.

Nowak said team members had begun interviewing former suspects held and subsequently released by U.S. authorities in efforts to establish conditions in the prisons and their exact locations.