The UN mission to Iraq called Tuesday for an impartial probe into the deaths of 52 Iranian exiles as the group they belonged to warned of an imminent troop offensive.
The deaths of the members of the People's Mujahedeen Organisation of Iran (PMOI) were condemned internationally but the UN and Western governments have been careful not to assign blame amid wildly differing accounts of what happened.
And Iraqi Prime Minister Nuri al-Maliki has set up an investigation in the aftermath of Sunday's unrest, with findings expected within days.
A UN statement issued on Tuesday, following a visit by a team of diplomats a day earlier, confirmed previously reported tolls of 52 dead, and said it was informed by PMOI members at Camp Ashraf, north of Baghdad in Diyala province, that seven members were still missing.
"The (UN) delegation witnessed 52 bodies in a makeshift morgue," the 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".
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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.
An umbrella body of Iranian opposition groups, of which the PMOI is a member, claimed on Tuesday that Iraqi troops had blocked off Ashraf in order to kill the remaining residents.
The Paris-based National Council of Resistance of Iran said in a statement that the soldiers aimed to "massacre the rest of the inhabitants of Ashraf".
Sunday's events follow two mortar attacks earlier this year on Camp Liberty, located on a former US military base on the outskirts of Baghdad, in which at least eight people were killed.
Around 3,000 members of the group, which is also known as the Mujahedeen-e-Khalq (MEK), were moved from Ashraf last year to Liberty, but about 100 stayed on at the old camp to deal with remaining property and goods.
Former Iraqi dictator Saddam Hussein allowed the rebel MEK to set up the camp during his war with Iran in the 1980s.
The MEK was founded in the 1960s to oppose the shah of Iran, and after the 1979 Islamic revolution that ousted him it took up arms against Iran's clerical rulers.
It says it has now laid down its arms and is working to overthrow the Islamic regime in Iran by peaceful means.
Britain struck the group off its terror list in June 2008, followed by the European Union in 2009 and the United States last year.