The Sadr movement has clashed with government forces
A relative and key follower of radical Shia cleric Moqtada Sadr has been shot dead in the Iraqi city of Najaf.
Riyad al-Nuri, the director of the cleric's office in Najaf, was killed by unidentified gunmen near his home as he returned from Friday prayers.
Police imposed an indefinite curfew in the city following the shooting.
The Sadr movement was involved in violent clashes with government forces after PM Nouri Maliki ordered a crackdown on militias last month.
Hundreds have died in clashes, mainly in the capital Baghdad and in Basra in the south.
An aide to Moqtada Sadr, Haider al-Turfi, told AFP news agency: "When [Mr Nuri] arrived from the prayers, they opened fire on him, killing him instantly."
Najaf police chief Maj Gen Abdul Karim Mustafa said an indefinite curfew had been imposed in the city.
Police have set up blockades and ordered shops to close.
Moqtada Sadr's spokesman in Najaf, Salah al-Obeidi, said the continued US presence in Iraq was to blame for the killing.
He said the cleric's message was to stay calm and not be "dragged into other's plots".
Mr Nuri was the cleric's brother-in-law.
Earlier, US forces said they had used an unmanned drone aircraft to kill six Shia militant fighters in a missile strike in the Sadr City area of Baghdad.
The US said the group had rocket-propelled grenade launchers and mortars.
In Basra, the British military said a further six people were killed in an air strike.
Spokesman Maj Tom Holloway said a US aircraft had identified a group of militants in the northern al-Hayaniyah district.
Basra saw the first of the clashes after Mr Maliki ordered his crackdown.
It has been quieter since Moqtada Sadr ordered his Mehdi Army militia off the streets.
In contrast, Baghdad has seen increasing violence this week, particularly in Sadr City.
Sadrists have said they are under siege from Iraqi and US forces and have threatened to break the general ceasefire Moqtada Sadr ordered last August.
On Monday, Mr Maliki threatened to exclude the radical Shia cleric's movement from politics unless he disbanded the Mehdi Army.
In other violence on Friday, a rocket struck the Palestine Hotel in central Baghdad, killing three civilians and wounding a number of others, Iraqi security officials said.
The hotel became famous as a journalists' centre during the 2003 US-led invasion but is now largely empty.