Assailants fired dozens of mortars and rockets at an Iranian exiles camp in a dawn attack that killed five people on Saturday, the first deaths from violence since they resettled near Baghdad last year.
It was not immediately clear who was behind the assault on Camp Liberty, a former US military base on the western outskirts of the Iraqi capital housing about 3,000 members of the People's Mujahedeen of Iran.
The United Nations mission in Iraq called for an immediate probe and said monitors were following up on the deaths, while the UN High Commissioner for Refugees condemned what he said was a "despicable act of violence".
Five members of the People's Mujahedeen were killed by the mortars and rockets, according to two Iraqi security officials who spoke on condition of anonymity.
The attack also wounded at least 40 members of the group, along with three Iraqi policemen, they said.
The Mujahedeen-e-Khalq (MEK), whose leadership is based in Paris, said in a statement that six people killed and 50 wounded.
The mortars struck Camp Liberty where residents from the MEK were moved last year, on Iraq's insistence, from their historic paramilitary camp of the 1980s -- Camp Ashraf.
One Iraqi security official said about 40 rockets and mortars were fired into the camp, and the MEK gave a similar figure.
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, said Iraqi officials had told the United Nations that "all those who were injured were hospitalised immediately."
Meanwhile, UN High Commissioner for Refugees Antonio Guterres issued a statement condemning the attack.
"I call on the Iraqi government to do everything it can to guarantee security to the residents," he said. "The perpetrators must be found and brought to justice without delay."
Camp Ashraf was the base now-executed dictator Saddam Hussein permitted the group to establish in Diyala province in the 1980s, during Iraq's eight-year war with Iran.
They are in the process of being resettled, and a US official said in October that Washington and several European countries had agreed to take them in.
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 US 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 Mujahedeen-e-Khalq has no support in Iran, and no connection to domestic opposition groups.