Violence has erupted at a camp in Iraq for dissident Iranians, who say dozens of residents have lost their lives.
The Mujahideen-e Khalq group accused Iraqi forces of attacking Camp Ashraf north-east of Baghdad, killing at least 52 of its members - some shot in the head at close range.
However Iraqi officials said no soldiers entered the camp.
UN representatives have condemned the bloodshed and urged Iraq swiftly to establish the facts.
A statement from the UN mission in Iraq also called on Iraqi officials to ensure security for residents at the camp and urged an end to the violence so medical help could reach the wounded.
"The only thing we can confirm is there are a lot of casualties,'' Eliana Nabaa, a spokeswoman for the UN mission to Iraq was quoted as saying.
The population of Camp Ashraf, once home to more than 3,000 members of the Mujahideen-e Khalq (MEK), was believed to have dwindled to about 100 before the violence.
In recent years, Iraqi authorities have been trying to dismantle the camp and eject the group, which has been based in Iraq since the 1980s.
Accounts of the violence given by MEK spokesmen and different local officials vary.
MEK officials said Iraqi forces attacked Camp Ashraf early on Sunday, firing mortar rounds.
Some Iraqi reports confirmed that mortar rounds had been fired without identifying their origin, but said the deaths came during subsequent clashes. Others said the blasts heard at the camp were caused by oil and gas canisters exploding.
Another source said there had been a raid on the camp overnight, but put the death toll at 19.
One source said Iraqi security forces opened fire after a crowd stormed a post at the camp entrance, wounding about 50 people, Reuters reported.
Local hospitals reported that three Iraqi soldiers were killed and four wounded in the violence, AFP said.
MEK officials sent out photographs and a video of people said to have been shot in the head during the clashes, but the images have not been independently verified.
A spokesman for the Iraqi prime minister confirmed there had been deaths among camp residents, but said initial investigations suggested they had resulted from infighting, AP reported.
Prime Minister Nouri Maliki has ordered an investigation, Reuters said.
Camp Ashraf was set up in the late 1980s to launch raids on Iran. It was welcomed by Iraq's then president, Saddam Hussein, who was fighting a war against Iran.
Most members of the MEK - also known as the People's Mujahedin Organisation of Iran (PMOI) - were moved to a new camp last year.
Iran considers the MEK a terrorist group
The group was removed from the US State Department's list of terrorist organisations last year.