The United Nations mission to Iraq called Tuesday for an impartial probe into the deaths of 52 members of an Iran opposition group in what it called an "atrocious crime".

A UN statement confirmed previously reported tolls of 52 dead, and said it was informed by members of the People's Mujahedeen Organisation of Iran (PMOI) at Camp Ashraf, north of Baghdad in Diyala province, that seven members were still missing.

The PMOI deaths were condemned internationally, but the UN and Western governments have been careful not to assign blame amid wildly differing versions of what happened.

Prime Minister Nuri al-Maliki has set up an investigation in the aftermath of the unrest.

"The (UN) delegation witnessed 52 bodies in a makeshift morgue," the UN statement said.

"All the deceased appeared to have suffered gunshot wounds, the majority of them in the head and the upper body, and several with their hands tied.

"The delegation also saw several damaged buildings, including one burnt, and was shown quantities of explosives."

UN deputy special envoy Gyorgy Busztin called on Iraq "to ensure that a thorough, impartial and transparent investigation into this atrocious crime is conducted without delay and that the results of the investigation are made public".

Iraqi officials and the PMOI have offered conflicting narratives of how the 52 died.

A senior Iraqi police officer, speaking on condition of anonymity, told AFP that the deaths appeared to be caused by infighting, and said investigators found a "huge amount of TNT and explosive materiel inside cars, houses and heavy machinery".

Iraqi officials insist that no soldiers entered Ashraf, and that explosions were triggered by mortar fire or the detonation of a barrel of oil or gas.

Those accounts are sharply contested by the PMOI, which charges that Iraqi forces entered Ashraf, killed 52 of its members and set fire to the group's property and goods.

It says Iraqi forces carried out a "massacre", and the group's members at Liberty, another camp for the group near Baghdad, began a hunger strike on Monday, a spokesman said.

Sunday's events follow two mortar attacks earlier this year on Liberty in which at least eight people were killed.

Around 3,000 members of the group, which is also known as the Mujahedeen-e-Khalq, were moved from Ashraf last year to Camp Liberty, on a former US military base on the outskirts of Baghdad, but about 100 stayed on at the old camp to deal with remaining property and goods.