Sadr threatened an "open war" last Saturday unless Maliki called off the campaign against his fighters in Baghdad, the southern oil hub of Basra and other Shi'ite areas.
Although Basra has since become quieter, fighting has continued in Sadr City.
A general uprising by the militia could unravel months of security gains in Iraq while U.S. soldiers are drawing down.
In a statement last Saturday, Sadr said he was giving the government a "final warning" or face an "open war until liberation". On Friday, Sadr made clear his battle was against U.S. forces, whom he has demanded leave Iraq.
"My brothers in the Mehdi Army and the Iraqi forces, be a single hand and stop shedding each other's blood. Support all types of resistance in order to have a safe Iraq," Sadr said.
His comments were the latest example of how the cleric, who has millions followers among Iraq's poor urban Shi'ites, keeps everyone guessing. Sadr himself has not appeared in public in a year and the U.S. military says he is living in Iran, where he is believed to be taking advanced Islamic studies.
Sadr launched two uprisings against U.S. forces in 2004.
His movement then entered politics and backed Maliki's rise to power in 2006. But the youthful Sadr split with Maliki a year ago when the prime minister refused to set a timetable for the withdrawal of U.S. troops from Iraq.
Maliki has threatened to bar Sadr's movement from provincial elections on October 1 if he does not disband the Mehdi Army.
Sadr's mass movement, which boycotted the last local elections in 2005, is expected to perform well at the expense of parties that backed Maliki, especially in the Shi'ite south.
Many of his followers see the crackdown as an effort to sideline them before the polls and protect groups that support Maliki. The government says the campaign is intended to restore the rule of law in militia-held areas.
