The People's Mujahedeen of Iran (MEK) said in a statement that the decision to carry out the attack was taken late January by Iran's national security council on Khamenei's orders.
It said that the 40 attackers and the rockets and mortars and other equipment they used in Saturday's attack were ferried to the camp by vehicles belonging to the Iraqi police and the Iraqi interior ministry.
The group of attackers was made up of members of the Shiite militant group Ketaeb Hezbollah and "technical agents" from Iran's elite al-Quds, said the statement by the MEK, whose leadership is based in Paris.
The MEK said Tuesday that seven people were killed in the attack on Camp Liberty, a former US military base near Baghdad housing about 3,000 members of the MEK.
The MEK, which gave an initial toll of six dead and around 100 wounded, said several more of its members remained in a serious condition.
No-one has claimed responsibility for the attack but MEK leader Maryam Radjavi on Monday denounced what she called the "crimes of the Iranian state".
She noted that the camp was inside a giant military zone and that the perpetrators could only have got inside with help from "within the Iraqi government".
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.