US and Iraqi troops seized an aide to Shia cleric Moqtada al-Sadr last night.
Dealing with Sadr and his Mehdi Army militia is a burning issue for the Americans and Shia Prime Minister Nuri al-Maliki as they prepare what many see as a last-ditch effort to rein in sectarian violence that is pushing Iraq into civil war.
The US military said in a statement that Iraqi special forces backed by American advisers seized an unnamed man they described as a death squad leader wanted for kidnap, torture and murder and linked to fugitive Shia warlord Abu Deraa.
Aides to Sadr said the man held was Abdul-Hadi al-Darraji, a prominent media spokesman for their movement. An official in Sadr's political office branded his detention a deliberate "provocation" but said they would not respond with violence.
It was reported that the aide was arrested with two cousins, that a guard was shot dead during the arrest, and that the two others detained had since been released.
Sadr, a young, populist cleric with a mass following and some backing from Shia Iran, is a key ally of Mr Maliki, who has been criticised by Washington and leaders of the once dominant Sunni Arab minority for failing to disarm the Mehdi Army.
Mr Maliki, however, has said this month he will crack down on Shia militias and said some 400 Mehdi Army members had been arrested in mainly Shia southern Iraq over recent days.