Iraqi Shia leaders linked to the radical cleric, Moqtada Sadr, have attacked their former government ally, Prime Minister Nouri Maliki.
Moqtada Sadr (left) and Nouri Maliki used to be allies
They accused Mr Maliki of bowing to US demands and sanctioning US attacks on Mr Sadr's Mehdi Army militia.
Mr Maliki has said the militia must purge its ranks of criminals.
Dozens of people have died in recent fighting between Iraqi forces and Mehdi Army militiamen, amid signs of a growing rift between the Shia groups.
In April, six cabinet ministers loyal to Mr Sadr quit their posts in protest at the government's refusal to demand a deadline for the withdrawal of US troops.
Support from Mr Sadr's bloc was critical to securing Mr Maliki's appointment as prime minister last year.
The Mehdi Army militia and its allies within the fledgling Iraqi security forces have been accused of operating sectarian death squads, targeting Iraqi Sunnis.
The militia's stronghold of Sadr City, a vast slum in eastern Baghdad, was the focus of a major US military operation in late June.
On Saturday, Mr Maliki said the Mehdi Army had been infiltrated by criminals and by members of the Baath Party of former Iraqi leader Saddam Hussein.
Mr Sadr's supporters said Mr Maliki's comments effectively gave US forces a "green light" to attack the Mehdi Army militia.
An aide to Mr Sadr, Sheikh Ahmed al-Shaibani, told the Associated Press news agency Mr Maliki "should not forget that his government was backed" by the Sadr movement.
He said Mr Maliki had tried to shore up his government with the attack on Mr Sadr's group.
According to Mr Shaibani, Mr Maliki's comments indicated he was ready to implement the US agenda of "ending the Mehdi Army militarily and politically".