BAGHDAD, (Reuters) – U.S. and Iraqi troops seized a prominent spokesman for Shi’ite cleric Moqtada al-Sadr on Friday, confronting a movement that has a key role in the ruling coalition but is accused by Washington of running death squads.

The midnight raid near Baghdad’s Sadr City district, which Sadr’s aides angrily called an “American provocation”, came as Defense Secretary Robert Gates flew in to the southern city of Basra to meet the commander in Iraq, General George Casey.

Dealing with Sadr and his Mehdi Army militia is a burning issue for U.S. forces and Shi’ite 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.

Gates said Iraq was at a “pivotal moment” and failure would be a “calamity” for U.S. interests.

Sadr, a young populist cleric with a mass following and some backing from Shi’ite Iran, is an ally of Maliki, who has been criticised by Washington and leaders of the once dominant Sunni Arab minority for failing so far to disarm the Mehdi Army.

Maliki, however, has said this month he will crack down on Shi’ite militias and said 400 Mehdi Army members had been arrested in mainly Shi’ite southern Iraq over recent days.

Friday’s move appeared to be part of a campaign of targeted operations which senior Shi’ite politicians have told Reuters are being mounted against key figures in the Mehdi Army. It was not clear which was the main suspect among at least three people arrested, including Abdul-Hadi al-Darraji, a spokesman for Sadr.

Government spokesman Ali al-Dabbagh said Darraji’s arrest was “not against the Sadrists” as a political movement but due to security concerns about Darraji, who would be released if an investigation cleared him. The U.S. military did not name the main suspect, who it said was held with two others.

In a statement that said Iraqi special forces backed by American advisers conducted the operation, it said it arrested a death squad leader wanted for kidnap, torture and murder and said he was linked to fugitive Shi’ite warlord Abu Deraa.

One senior source in the Sadr movement said Darraji may not have been the main target as he was detained with “guests”, one of whom may have been a senior Mehdi Army commander.

General Casey said he could not confirm Darraji was among those arrested, but he added: “I think it is indicative of the prime minister’s and the government’s commitment to target all those who break the law.”

“We have picked up probably five or six death squad leaders here in the last three to four weeks at very very high level,” Casey told reporters at a joint news conference with Gates.

Shi’ite militias, along with Sunni insurgent groups, are blamed for thousands of killings in the past year — the United Nations says more than 34,000 civilians died in 2006. Dozens of people are found tortured and shot in Baghdad every day.

Condemning the arrest, the leader of the Sadrist bloc in parliament, Nassar al-Rubaie, told Reuters: “This is a great provocation to the Sadrists but it will not drag us into doing what the Americans want, which is to create chaos in Iraq.”

Government spokesman Dabbagh said the operation had Maliki’s full backing: “This is not against the Sadrists. It is against people with security question marks against them.”

Rubaie, calling Darraji a “moderate”, said other government officials told him they had been unaware of the plans for the raid. But Dabbagh stressed that it was a “joint operation”.

The U.S. military said: “In an Iraqi-led operation, special Iraqi army forces captured a high-level, illegal armed group leader during operations with Coalition advisers.”

It said the main suspect was accused of leading “punishment” activities — an apparent reference to informal courts meting out rough justice according strict interpretations of Islam.

After criticism from Washington, Maliki has announced that the coming crackdown in Baghdad, backed by most of the 21,500 American reinforcements being sent by President George W. Bush, will tackle Shi’ite militias as well as Sunni insurgents.