U.S. Defense Secretary Ash Carter said on Monday the Pentagon would dispatch 560 additional troops to help Iraqi forces retake the northern city of Mosul in an offensive planned for later this year.
Sadr, who rose to prominence when his Mahdi Army battled U.S. troops after the 2003 invasion, posted the comments on his official website after a follower asked for his response to the announcement.
"They are a target for us," Sadr said, without offering details.
Sadr, who commands the loyalty of tens of thousands of supporters, is also leading a protest movement that saw demonstrators storm Baghdad's heavily fortified Green Zone government district twice this year, hampering parliament for weeks.
The new troop deployment, which is expected to happen within weeks, would raise the number of U.S. forces in Iraq to around 4,650, far below the peak of about 170,000 reached during the nearly nine-year occupation.
Other Shi'ite militias, particularly those backed by Iran, have made similar pledges to attack U.S. soldiers in the past year, but the only casualties since American forces returned to Iraq to battle Islamic State two years ago have come at the hands of the Sunni militant group.