In a sermon read by an aide during Friday prayers in Baghdad's militia stronghold of Sadr City, the cleric also urged Iraqi soldiers and policemen "not to support the occupiers in combating your brothers."
Al-Sadr issued a "final warning" to the government Saturday to halt its crackdown against the Mahdi Army or face an "open war until liberation."
The statement on al-Sadr's Web site singled out the Iraqi government led by fellow Shiite Nouri al-Maliki, accusing him of selling out to the Americans. Friday's sermon appeared to be an attempt to ease tensions.
Al-Sadr - who is believed to be in Iran - called on worshippers to remain patient and united.
"If we have threatened with an open war until liberation, we have meant by it a war against the occupier," said the sermon, which was read by the mosque's imam Sheik Hassan al-Edhari.
"I call upon my brothers in police, army and Mahdi army to stop the bloodshed," the sermon said. "We should be one hand in achieving justice, security and in supporting the resistance in all of its forms."
"There will be no war between our Iraqi brothers, whatever their sect or ethnicity," it said.
Military operations launched by al-Maliki late last month in the southern city of Basra led to daily clashes between militia fighters and U.S.-backed Iraqi troops, focused mainly in sprawling Sadr City. Militiamen also fought Iraqi security forces to a virtual standstill last month in Basra before an Iranian-supervised truce.
Years of intense fighting between Shiites and Sunnis had only recently ebbed.
Senior al-Sadr aide Hazim al-Aaraji told The Associated Press that the new message was also intended to be read in Basra but a delegation from the movement was prevented from entering the city. They read it in the southern city of Nasiriyah instead, he said.
"The statement is for all but we aimed for it to be read in Basra and Sadr City because of the violence going on in these two areas," he said. "The aim was to ease the situation in Basra and Sadr City."
The Sadrists accuse al-Maliki, a political rival, of trying to sideline them ahead of expected provincial elections in the fall. They appear divided over whether to launch a full-scale fight against U.S.-led forces or focus on political efforts.
A decision by al-Sadr to lift a nearly eight-month cease-fire would jeopardize recent security gains and threaten an increase of attacks against U.S. troops.
On Thursday, al-Maliki vowed that the crackdown on Shiite militias had won broad political support from Sunni, Shiite and Kurdish political parties and would continue.
The U.S. military said American and Iraqi forces killed 10 militants in overnight clashes in northeastern Baghdad. Most were killed by Hellfire missiles launched from Apache helicopter gunships against groups of militants preparing to fire on U.S. and Iraqi forces from different parts of the district, according to a statement.
Local hospital officials, speaking on condition of anonymity because they were not authorized to speak to the media, said seven people, including two women, were killed and 45 others were wounded in the clashes that were centered in the Sadr City district.
Witnesses said the clashes ended in the early hours Friday.
Meanwhile, the military said a U.S. soldier was killed in a roadside bombing south of Baghdad.
The attack raised the American death toll in April to 39, the highest rate of death for troops in Iraq since September, when 65 Americans were killed, according to an Associated Press tally.
In all, at least 4,051 members of the U.S. military have died since the Iraq war started in March 2003, the AP tally showed.
Associated Press writers Selcan Hacaoglu and Hamid Ahmed contributed to this report.
© 2008 Associated Press