Iraqi troops have cordoned off the Basra office of Shi'ite cleric Moqtada al-Sadr's followers, preventing them from holding Friday prayers.
Iraqi Interior Ministry spokesman Major General Abdul-Karim Khalaf said the operation is only aimed at recovering offices he said were unfairly occupied by political groups.
Sadr supporters say they have been given 48 hours to leave the premises.
The standoff comes nearly a month after Iraqi Prime Minister Nouri al-Maliki launched a crackdown against Sadr's Mahdi Army militia in Basra, sparking violent clashes across cities in the south and in the capital, Baghdad.
The U.S. military is warning that it has intelligence that al-Qaida in Iraq militants are planning suicide bombings in Baghdad in the near future. The statement, issued Friday, is an unusual warning by the U.S. military, citing what it calls "credible intelligence."
The warning notes that a recently stolen ambulance could be used in an attack.
The U.S. military also announced that a U.S. soldier was killed north of Baghdad Friday when a roadside bomb struck his vehicle.
On Thursday, in northern Iraq, a suicide bomber targeted a funeral near Kirkuk, killing at least 50 people and wounding more than 20 in one of the deadliest attacks in recent months.
Some information for this report was provided by AFP, AP and Reuters.