It is the first time Sadr, who is himself judged by critics as close to Tehran, has publicly stated that Asaib Ahel al-Haq, or the League of the Righteous, is supported by the Islamic Republic.
The cleric said that the Shiite militia, which is blamed for the killing of US troops, had only recently decided to lay down their arms because a political standoff in Baghdad has raised the spectre of early elections. The group was also behind the kidnap of a British IT consultant and his four bodyguards.
"I have asked the people who are in charge of them in the Islamic Republic to change the name of Asaib, and change their dual leadership," Sadr said in a written response to a letter from a follower, published by his office on Sunday. "But these people refused."
Washington has long blamed Iran for training and equipping Shiite militias, including Asaib Ahel al-Haq, that have carried out attacks against US and Iraqi soldiers, charges Tehran denies. Sadr did not specify what he meant by the group's "dual leadership" but Asaib Ahel al-Haq is jointly led by the brothers Qais and Laith al-Khazali.
The cleric long ran his own feared Mahdi Army militia, and while that has been deactivated as a violent force, the offshoot Promised Day Brigade is seen as close to Sadr. "They (Asaib Ahel al-Haq) handed over their weapons to be part of the political process," Sadr said, referring to Qais al-Khazali's December 26 remarks that the group would join the political process.
Sadr said those weapons were used to kill "honest people", charging the organisation with killing Iraqi soldiers and policemen, as well as Saleh al-Ogayly, an MP who was killed by a booby-trapped motorcycle in the Shiite bastion of Sadr City in Baghdad in October 2008.
"Today, the opportunity of an election has come, and their intentions have become clear," Sadr said.
He was referring to political discord in Iraq between the Shiite-led government and the key Sunni-backed Iraqiya party, with several groups, including the parliamentary bloc loyal to Sadr, calling for early elections to resolve the standoff.