All prisoners in Iraq must be freed

"The USA has announced that it intends to continue to hold, without charge, between 4,000 and 5,000 detainees without clarifying on what legal basis it will do so," the group said.

"Yet if, as the UN resolution (1546) proclaims, occupation effectively ends with the handover, then international humanitarian law requires that all prisoners of war, detainees and internees must be released by the occupying powers," it said in a statement.

The US-led coalition's civil administrator Paul Bremer on Monday handed a document to Iraqi leaders which formally ended -- two days earlier than expected -- the US-led occupation of Iraq and returned sovereignty to Iraqis.

Amnesty said it had received no reply from a June 9 letter it wrote to the US envoy to the United Nations, John Negroponte -- the future ambassador to Iraq -- expressing serious concern that UN resolution 1546 fails to clarify what would happen to the thousands of prisoners held by the occupying powers.

"Any further detentions by the US and other members of the multinational force after the handover would be unlawful," the rights group said.