Dozens of mortars and rockets fired on a camp housing Iranian dissidents near Baghdad killed five members of the opposition group early on Saturday, Iraqi security officials said.
The United Nations called for an immediate investigation and said monitors were following up on deaths, the first confirmed fatalities as a result of violence at the group's new camp since they moved there last year.
Five members of the Mujahedeen-e-Khalq (MEK) were killed by the mortars and rockets, two Iraqi security officials said on condition of anonymity. Between 39 and 40 members of the group were wounded, along with three Iraqi policemen.
The MEK, whose leadership is based in Paris, said in a statement that six people were killed and 50 wounded.
One Iraqi security official said around 40 rockets and mortars were fired into the camp, while the MEK said 35 were launched.
It was not immediately clear who was responsible for the attack.
The United Nations said its special envoy Martin Kobler had asked Iraqi authorities to "promptly conduct an investigation into this," and added, "we have our monitors on the ground to follow-up."
Eliana Nabaa, spokeswoman for the UN mission in the country, said Iraqi officials had told the United Nations that "all those who were injured were hospitalised immediately."
The mortars struck at a transit camp known as Camp Liberty where some 3,000 residents from the MEK were moved last year, on Iraq's insistence, from their historic paramilitary camp of the 1980s -- Camp Ashraf.
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 Tehran through peaceful means.
Britain struck the group off its terror list in June 2008, followed by the European Union in 2009 and the United States in September 2012.
The state department holds the group responsible, however, for the deaths of Iranians as well as US soldiers and civilians from the 1970s into 2001.
The MEK has no support in Iran, and no connection to domestic opposition groups.