The United Nations says it's concerned about overcrowded prisons and the treatment of detainees in Iraq.

Senior U.N. envoy Staffan de Mistura says the issue will be a major challenge as the United States prepares to turn over control of thousands of inmates in its custody to Iraq's government.

The transfer is required under a security pact with the U.S. recently approved by Iraq's parliament.

De Mistura says reports of torture need to be thoroughly investigated. He also has called on the Iraqi government to speed up legal reforms and strengthen the judicial system.

He says improvements in the rule of law are necessary to ensure security gains are sustainable.

His remarks came Tuesday as the U.N. released its latest human rights report for Iraq.

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