The UN human rights office in Iraq said in a report on Thursday that it had received serious allegations about elements in the police and special forces and "their apparent collusion with militias in carrying out human rights violations".
Allegations that death squads operate in the country had grown stronger after the discovery by US-led forces and the Iraqi security forces in January of a suspicious group operating within the Iraqi interior ministry, it said.
Twenty-two men, dressed as special police commandos, were caught when driving with a man who was allegedly about to be executed, it said.
"This reaffirms the urgent need for the government to assert control over the security forces and all armed groups," the UN report said.
Numerous summary executions had taken place in and around Baghdad during the period, reportedly by armed militias, thereby further fuelling sectarian tensions and violence, it said.
Conditions and the legality of detention in Iraq remained of particular concern, it added.
"The [UN] Human Rights Office also continues to receive regular allegations and evidence of torture in detention centres, particularly [those] not operated or controlled by the ministry of justice," it said.
The UN also said military operations by US-led and Iraqi forces, especially in western al-Anbar province, had raised concerns of "excessive use of force", mistreatment and theft during raids, as well as the demolition of houses.
And minority groups, including some of the estimated 34,000 Palestinian refugees living in Iraq, are being subjected to increasing detention, torture and discrimination because of their alleged links to foreign Arabs supporting the fight against US-led forces, it added.