A Shia cleric loyal to anti-US cleric Moqtada al Sadr has warned that the 300 US military advisers en route to Iraq will be attacked.
In a sermon from Baghdad's Sadr City district, Nassir al Saedi threatened what he called "the occupier", saying: "We will be ready for you if you are back."
The warning comes after President Barack Obama announced the deployment of US military advisers, made up mostly of special forces.
Their aim is to help the Iraqi government in its fight against the Islamic State of Iraq and Syria (ISIS), the group of Sunni extremists who have seized cities and towns across northern Iraq.
Mr Obama is deploying 300 military advisers as pressure mounts on Mr MalikiAl Saedi's threats highlight a potentially dangerous secondary front for US forces heading to Iraq. Moqtada al Sadr's militia fought the Americans in at least two rounds of street warfare during the eight years US troops were on the ground there.
Meanwhile, UN Secretary General Ban Ki-moon on Friday urged the Iraqi government and its supporters not to retaliate against Sunni communities in revenge for ISIS' "barbaric attacks".
He warned that any possible airstrikes against the Islamist militants "might have little lasting effect or even be counter-productive" if Iraq's feuding communities fail to unite against the terrorists.
Embattled Iraqi Prime Minister Nouri al Maliki has asked the US to launch airstrikes to contain the militant group.
On Thursday, Mr Obama said "targeted" military action remained an option in Iraq, but said US forces on the ground would not take part in combat operations.
Mr Maliki, a Shia Muslim, has come under mounting criticism for failing to halt ISIS in its tracks.
Iraq's Shia religious leader Grand Ayatollah Ali al Sistani, who had previously called for all Iraqis to join the fight against the militants, has now called for a new "effective" government.
He urged the newly-elected parliament to start work without delay after the results of recent elections were ratified by Iraq's federal court.