Fiery Shiite cleric Moqtada al-Sadr sharply criticised an offshoot of his movement on Sunday, accusing them of killing Iraqi soldiers and policemen and being beholden to neighbouring Iran.

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 2007 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."

One of Asaib Ahel al-Haq's leaders later dismissed Sadr's accusations of political expediency.

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.

Sadr 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 national unity 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.

In a speech in Baghdad's Tahrir Square on Sunday, however, Qais al-Khazali said that Asaib Ahel al-Haq would not accept any positions in government, and would work from the opposition if elected.

"It is necessary to discuss the major mistakes the political process is suffering from, such as the lack of existence of an opposition monitoring the work of the government," he said.

"The sons of the resistance, they take this responsibility on their shoulders," he added, referring to his group. "They agree to sacrifice seats and posts offered to them, or those which could be offered to them in future."