The US-led force continues to hold far more prisoners than the Iraqi government, and most are individuals picked up in mass arrests and detained for "imperative reasons of security," the world body said on Monday.
"While progress in reviewing cases led to the release of hundreds, the overall number of detainees continued to increase due to mass arrests carried out during security and military operations," the UN mission said in its latest progress report on human rights, covering the period 1 September to 31 October.
"There is an urgent need to provide remedy to lengthy internment for reasons of security without adequate judicial oversight," it said.
The number of detainees now held by the US-led forces has climbed to 13,514, according to the latest Pentagon figures, up from about 6000 in June and 9600 in September.
That is far more than the 7577 in the custody of Iraq's Justice Ministry, the 3916 held by its Interior Ministry and the 342 juveniles in the hands of its Labour and Social Affairs Ministry as of 26 October, according to Iraqi figures.
The United Nations has repeatedly expressed concern about the large number of detainees being held in Iraq without apparent due process, alleging that thousands were being held for extended periods without charges or even preliminary reviews to determine whether charges were ultimately likely.
The US military insists Iraqi detainees are having their cases promptly reviewed, whether through referral to an Iraqi court or through the new prisoner review board.
The board, composed of both Iraqi and US officials, was set up in August 2004 to speed the review of individual cases.
The UN says mass arrests have
swelled the number of detainees
But the UN report said its standards violated both Iraqi and international laws governing the treatment of civilians.
And while the board is reviewing 200 to 250 cases a week, "resulting in some releases," the overall number of detainees is still climbing due to more mass arrests, it said.
The US-led force is able to detain Iraqis under an exception to the Geneva Conventions - which govern the treatment of civilians in wartime - granted by the 15-nation UN Security Council in June 2004.
The council last week voted unanimously to extend the exception through to the end of 2006.