The UN says US-led troops held 24,514 inmates in Iraq at the end of 2007
The UN Security Council should address serious concerns about the detention policies of the US-led forces in Iraq, Human Rights Watch (HRW) has said.
The New York-based group says thousands of Iraqis are being held indefinitely and without judicial review.
It claims that many inmates are subject to judicial review processes that do not meet international standards.
HRW says the US improperly uses Council resolutions which permit internment for "imperative reasons of security".
Separately, the group adds that there are also concerns about what is describes as widespread torture of detainees by the Iraqi authorities.
US and Iraqi officials have so far not commented on the claims by HRW.
HRW raises its concerns in a letter to UN Security Council members.
It argues that thousands of inmates in custody of the US-led troops should be provided due process under international human rights law.
"The Bush administration pushed the [UN] Security Council to declare that the US-led occupation of Iraq had ended in June 2004," said Joe Stork, HRW's Middle East deputy director
"And the end of occupation means that international human rights standards apply - judicial review, access to legal counsel and family members, and a fair trial," he said.
HWR also calls on Washington to allow observers from the UN Assistance Mission in Iraq (Unami) - as well as independent Iraqi experts - to visit US detention facilities.
The US-led troops were holding more than 24,000 people in Iraq at the end of last year, according to Unami.