Assailants fired dozens of mortars and rockets at an Iranian exiles camp in Iraq in an attack that killed five people, the first violent deaths since they resettled near Baghdad last year.
It was not immediately clear who was behind Saturday's assault on Camp Liberty, a former US military base on the outskirts of the capital housing about 3,000 members of the People's Mujahedeen of Iran, or the Mujahedeen-e-Khalq (MEK).
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 called a "despicable act of violence."
The US State Department labeled the assault a "vicious and senseless terrorist attack," and called on Iraq to probe the attack and enhance security at the camp.
Five members of the People's Mujahedeen were killed by the mortar rounds 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 and three Iraqi policemen, they said.
The US State Department put the toll at six dead and dozens injured.
The MEK, whose leadership is based in Paris, said in a statement that six people were killed and more than 100 wounded.
The mortar fire 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, near the Iranian border.
One Iraqi security official said about 40 rockets and mortar rounds 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," adding: "We have our monitors on the ground to follow up."
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Eliana Nabaa, spokeswoman for the UN mission, said Iraqi officials had told the United Nations that "all those who were injured were hospitalised immediately."
Camp Ashraf was the base that now-executed dictator Saddam Hussein allowed the group to establish in Diyala province in the 1980s, during Iraq's eight-year war with Iran.
The residents are in the process of being resettled, and a US official said in October that the United States 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 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 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 to 2001, and in its note about delisting the MEK it stressed that it had not forgotten the group's militant past.
A senior US official also said at the time that Washington does "not see the MEK as a viable opposition."
In Washington on Saturday, at a convention of pro-democracy Iranian-Americans, former New York mayor Rudy Giuliani called on the United States to take in the camp residents.
"These people can all be removed within hours," said Giuliani. "Planes can be sent immediately. They can be here within a day. We have done far more difficult things than that. It's only about 3,000 people."
Another guest speaker at the convention, former national security advisor General James Jones, said: "Camp Liberty has a name that is not worthy of a prison, which it has become.
"It is the responsibility of all countries of goodwill to ensure that the victimization of the Ashraf refugees is not perpetrated and that this debacle comes to a swift and just conclusion."