BAGHDAD, Jan 20 (Reuters) - Shi'ite cleric Moqtada al-Sadr's political movement accused Washington on Saturday of trying to provoke a confrontation, after the arrest of a top aide to the militia leader, who is an ally of Iraq's Shi'ite prime minister.

Abdul-Hadi al-Darraji, a spokesman for Sadr, was among at least three people arrested by U.S. and Iraqi troops in a midnight raid on Sadr City, a stronghold of Sadr's Mehdi Army in northeast Baghdad where U.S. forces rarely venture.

Abdul Mahdi Mtiri, a member of the Sadrists' political committee, said Iraqi officials had promised Darraji would be released. "We don't know how serious this promise is because so far he has not been released," Mtiri told Reuters.

Government spokesman Ali al-Dabbagh told Iraqiya state television he did not expect him to be released on Saturday.

"The matter is not in the hands of the Iraqi government. The Americans arrested him and they're investigating him and when they're finished they will release him," said Dabbagh, who said on Friday the operation had the full backing of Prime Minister Nuri al-Maliki.

Dealing with Sadr and his Mehdi Army militia is a burning issue for U.S. forces and Maliki, a Shi'ite Islamist, as they prepare what many see as a last-ditch effort to curb the sectarian violence that is pushing Iraq towards civil war.

"We know the truth behind this arrest is the Americans want to target the Sadrists and they want to draw the Sadrists into a confrontation with the American troops," Mtiri said.

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

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, vowed this month to crack down on Shi'ite militias and said 400 Mehdi Army members had been arrested in mainly Shi'ite southern Iraq in recent days.

Dabbagh said on Friday Darraji's arrest was "not against the Sadrists" as a political movement but motivated by security concerns about Darraji, who would be released if an investigation cleared him. The U.S. military did not confirm he was among those arrested.

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.

In Washington, Bush's plan to send more troops faced opposition from newly empowered congressional Democrats who are pushing for a phased withdrawal from Iraq.

House of Representatives Speaker Nancy Pelosi accused Bush on Friday of playing politics with soldiers' lives, a charge the White House called "poisonous".

"The president knows that because the troops are in harm's way that we won't cut off the resources," Pelosi, head of the Democratic-led House, told ABC's "Good Morning America".

"That's why he's moving so quickly to put them in harm's way."

Bush, faced with opinion polls showing Americans strongly oppose a troop increase, is expected to defend his Iraq plan in his State of the Union speech on Tuesday.

The U.S. military announced three more combat deaths on Saturday. More than 3,000 soldiers have died since the March 2003 invasion.