Iraq's Sadr calls on supporters to target US troops
Sadr's remarks were made on his official site after he was asked to comment on US Defence Secretary Ash Carter's announcement last week that the Pentagon will deploy more troops to Iraq.
"They are a target for us," Sadr said without giving any details, Reuters reported.
Last week Carter announced that his country will send a further 560 US soldiers to help support the offensive to recapture Mosul from Islamic [IS] group.
The new troops will bring the total number of US soldiers in Iraq to around 4,650.
Sadr, who has the support of tens of thousands of Iraqis, had fought US troops during their 2003 invasion of Iraq with the Mahdi Army.
The Mahdi Army was disbanded in 2008 and replaced by the Peace Brigades, who fought under a government-led umbrella group to ward off IS militants from the Iraqi capital city of Baghdad in 2014.
The Shia cleric is also heading a protest movement against the Iraqi government calling for reform and an end to corruption.
His supporters broke into Baghdad's fortified Green Zone area in April, storming parliament.
Other Iran-backed Shia armed groups also pledged to fight US soldiers in Iraq, but the only casualties American forces have suffered since returning to the country in 2014 have been at the hands of IS militants.