"The investigation is under way," joint operations command spokesman Yahya Rasool said of the attack, which drew condemnation from the United States, United Nations and rights group Amnesty International.
According to Iraqi security sources, at least 15 rockets fired from an area west of Baghdad called Bakriya struck in and around Camp Liberty yesterday evening.
The former US military base near the international airport houses members of the People's Mujahedeen Organisation of Iran (PMOI), a group that originally opposed the shah but later fought the country's clerical rulers after the 1979 Islamic revolution.
The PMOI released photos said to show bodies of the victims killed in the attack, one of which showed 20 people lying on stretchers on the ground.
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An official at Yarmuk hospital in Baghdad said three of the dissidents, who are also known as the Mujahedeen-e-Khalq, were killed and 18 wounded, which would bring the total death toll to 23.
The toll could not be independently confirmed, and Rasool said only that two members of the Iraqi security forces were wounded by the rocket fire.
"The rockets struck areas around Liberty, which was not the only place targeted," he said.
Rasool said a truck mounted with rocket launchers was found in Bakriya, north of Camp Liberty.
"The people behind this attack are terrorist criminals who want to destabilise the country," he said, without naming any group or nation.
Rockets were launched from a truck towards the airport in late September, while the PMOI said Camp Liberty was targeted in a rocket attack in late 2013.
The PMOI sided with Saddam Hussein's Iraq during the war with Iran in the 1980s and was accused of helping suppress a Shiite uprising in 1991, but the 2003 US-led invasion brought to power leaders who have ties to Tehran and despise the group.