Six people were killed and dozens injured in a rocket attack on a camp for Iranian dissidents in Iraq on Saturday.
The Obama administration condemned the assault, which it described as a "vicious and senseless terrorist attack."
A woman was among those killed in the attack, which took place early on Saturday morning, said Shahriar Kia, a spokesman for the residents of Camp Liberty near Baghdad's international airport.
More than 100 people were wounded, some of whom are in critical condition, he added.
Camp Liberty residents are all members of the Mujahedeen-e-Khalq, an Iranian dissident group that seeks to overthrow Iran's theocratic regime.
"The United States condemns in the strongest terms the vicious and senseless terrorist attack," said State Department spokeswoman Victoria Nuland.
"We call on [the Iraqi government] to earnestly and fully carry out that investigation and to take all appropriate measures to enhance the security of the camp consistent with its commitment and obligation to the safety and security of the camp's residents," Mrs. Nuland said. "The terrorists responsible for this attack must be brought to justice."
The Obama administration is consulting with the Iraqi government and the United Nations Assistance Mission for Iraq (UNAMI) on the circumstances surrounding the attack.
UNAMI said it was informed about "mortar explosions" in Camp Liberty, which it said led to a number of deaths and injuries among the camp's residents. Iraqi policemen were also injured in the attack, it said.
Mr. Kia said that despite opposition from the exiles, the Iraqi government had removed concrete blast walls surrounding Camp Liberty, leaving the residents vulnerable.
"Removal of the protective [blast] walls was nothing but a filthy conspiracy for annihilation of all of the residents," he added.
MeK's deputy secretary-general, Massoumeh Maleck Mohammadi, was among those injured, said Shahin Gobadi, a Paris-based spokesman for the National Council of Resistance of Iran.
The special representative of the U.N. secretary-general for Iraq, Martin Kobler, "has asked the Iraqi authorities to promptly conduct an investigation," UNAMI said in a statement.
U.N. monitors are on the ground, it added.
U.N. High Commissioner for Refugees Antonio Guterres condemned the attack as a "despicable act of violence" and called on the Iraqi government to "do everything it can to guarantee security to the residents."
In September, the Obama administration removed the MeK from the State Department's list of foreign terrorist organizations. The decision was motivated in part by the group's cooperation to relocate from Camp Ashraf, its paramilitary base north of Baghdad, to Camp Liberty.
MeK's Paris-based leader, Maryam Rajavi, wrote to President Obama and U.N. Secretary-General Ban Ki-moon and urged them to transfer the 3,100 residents of Camp Liberty back to Camp Ashraf.
Camp Ashraf's residents were transferred to Camp Liberty under a deal brokered by the United Nations in December.
Iraqi strongman Saddam Hussein gave MeK refuge in the 1980s. After Saddam was overthrown in a U.S. invasion in 2003, U.S. military forces disarmed the dissidents, who had renounced violence in 2001.
The MeK was responsible for terrorist attacks in Iran in the 1970s that killed several U.S. military personnel and civilians, according to the State Department. The group denies any role in the deaths of U.S. military personnel.
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