The US is making use of prison ships to keep in custody those arrested in its so-called war on terror, a human rights group says.

According to a study carried out by the human rights group Reprieve, the US may have used as many as 17 ships as "floating prisons" since 2001, and prisoners are interrogated aboard the vessels and then rendered to other, often unknown, locations, wrote a Guardian article Monday.

Clive Stafford Smith, Reprieve's legal director, said "they choose ships to try to keep their misconduct as far as possible from the prying eyes of the media and lawyers. We will eventually reunite these ghost prisoners with their legal rights."

Washington denied the operation of the "floating prisons." A US navy spokesman, Commander Jeffrey Gordon, told the Guardian "there are no detention facilities on US navy ships."

But he added that it was a matter of public record that some individuals had been put on ships "for a few days" during what he called the initial days of detention.

According to the report, ships that allegedly have held prisoners include the USS Bataan and USS Peleliu, and a further 15 ships are suspected of operating around the British territory of Diego Garcia in the Indian Ocean, which has been used as a military base by Britain and the United States.

The United States is currently keeping many terror suspects in several rendition sites and secret prisons, known as "black sites", across the world. In the lack of any legal process, the prisoners have to undergo harsh interrogations.

RB/RA