The UN called on Iraq to immediately start an independent inquiry into a raid on an Iranian opposition camp that left 34 dead, after expressing "deep concern" over the incident on Saturday.

The United Nations Assistance Mission to Iraq (UNAMI) also said it would continue to help the Baghdad government find a permanent solution to deal with Camp Ashraf, which is home to around 3,500 Iranian exiles.

"UNAMI has expressed deep concern over the events and has repeatedly urged the government of Iraq to refrain from the use of force," a UN statement said.

"UNAMI has noted the initiative of the (government) to establish a commission of inquiry and expects such a commission to be independent and to start its work without delay."

The statement came a day after UN human rights chief Navi Pillay called for an independent inquiry into the April 7-8 raid on Camp Ashraf, which houses the People's Mujahedeen of Iran (PMOI).

Iraqi security forces raided the camp as tensions between the opponents of Iran's clerical regime and the Iraqi authorities reached new heights.

Iraqi government spokesman Ali Dabbagh denied the military carried out the killings and said Baghdad would hold its own investigation. Iraq said after the raid that the PMOI must leave the country by the end of the year.

The United Nations said in its statement that it would continue "to assist the government of Iraq in search of a permanent solution to the issues of Camp Ashraf."

The left-wing PMOI was founded in 1965 to oppose the shah of Iran but after the Islamic revolution in 1979 took up arms against the clerical regime. The group is on the US government terrorist list.

Camp Ashraf was disarmed following the US-led invasion of Iraq in 2003 and has become a mounting problem for Iraqi authorities since US forces handed over security for the camp last year.