Gunmen shot dead a top aide to Shiite cleric Moqtada al-Sadr in Iraq's holy city of Najaf on Friday, sparking anger among his followers as they battle government forces on two fronts.
Riyad al-Nuri, director of the Sadr movement's office in Najaf, was shot dead near his home as he returned from Friday prayers, Najaf police chief Major General Abdul Karim Mustafa said.
Police immediately imposed an indefinite curfew in the shrine city following the shooting, while Prime Minister Nuri al-Maliki strongly condemned the attack and the Sadr movement blamed U.S. forces "and others working with them."
A Sadr official in Najaf, Haider al-Turfi, said the gunmen were waiting for 37-year-old Nuri near his home in the city's eastern neighborhood of Al-Adala.
Sadr's spokesman in Najaf, Salah al-Obeidi, lashed out at the U.S. military.
"The occupation forces and others who are working with them are responsible for this assassination," Obeidi told AFP.
"Those who have done this want the situation in Iraq to be unstable and want the fighting between the Sadrists and Iraqi forces to continue," he added. "This action serves the interest of the occupier."
Nuri was a senior leader in the cleric's movement and his sister had been married to Sadr's brother Murtada who was killed in 1999.
The attack comes as Sadr's Mahdi Army militia is engaged in deadly clashes with Iraqi security forces in their eastern Baghdad bastion of Sadr City and in the southern port city of Basra.
Mahdi Army militiamen have been battling Iraqi troops in the two Shiite strongholds since March 25, when Maliki ordered a crackdown on militiamen in Basra.
At least 700 people have been killed in the fighting, which began in Basra but quickly spread to Shiite areas of Baghdad and other regions of Iraq.
The battles subsided after Sadr pulled his fighters off the streets on March 30, but fighting erupted in greater fury a week later in Sadr City when Iraqi and U.S. forces began new operations in the sprawling township.