BAGHDAD, Iraq — More than 3,000 fresh American troops arrived in Baghdad this weekend, in the first phase of a “surge” of forces to secure the streets of the capital.

The reinforcements came as the American military prepared to send home 25 troops in body bags after its deadliest day in the past two years.

About 3,200 paratroops of the 2nd Brigade, 82nd Airborne Division deployed as the first of a 21,500-strong task force ordered by President Bush to reinforce security in the city.

Their arrival is meant to ensure America controls the streets of Baghdad from rampant sectarian militias.

However, the bloodshed continued as the American military announced that 25 American troops had been killed on Saturday, including 12 when a helicopter crashed north of Baghdad.

A terrorist group linked to Al Qaeda claimed responsibility for downing an American military Black Hawk helicopter in Diyala province.

An American military spokesman said the cause of the crash was being investigated, but the crew had reported mechanical failure.

The Mujahedeen army claimed on a Web site that it used a ground-to-air missile to shoot down the helicopter near the town of Buhriz.

A suspected grenade, rocket, and assault rifle attack by militia fighters loyal to the Shiite cleric Moqtada al-Sadr against a provincial security bureau office in the southern city of Kerbala left a further five American soldiers dead.

The local governor said two American soldiers also had been kidnapped, but the American military said it had accounted for all troops involved.

Meanwhile, Mr. Sadr’s militia —believed to be responsible for much of the sectarian bloodshed in Baghdad — announced that it was returning to the political fold in an apparent bid to regain legislative influence ahead of a planned security crackdown.

His followers said they were lifting their political boycott after reaching a compromise in which a parliamentary committee would take up the group’s demands for a timetable for Iraqi forces to take over security and the withdrawal of American forces.

“We announce our return to Parliament. We will attend today’s session, and the ministers will resume their work to serve the people,” one of 30 lawmakers loyal to Mr. Sadr, Bahaa al-Araji, said.

He was speaking at a news conference attended by the Sunni Parliament speaker Mahmoud al-Mashhadani. Mr. Sadr also has six loyalist ministers in the 38-member Cabinet.

Mr. Sadr is a key supporter of Prime Minister al-Maliki, whose Shiite-dominated government has held power since May 25. The two previous missions largely failed because he blocked American attempts to go after the Mahdi army militia, the armed wing of Mr. Sadr’s organization.

But yesterday, Mr. Maliki predicted major changes in political stability, the economy, and public services this year.

Since Mr. Bush and Mr. Maliki announced the planned drive against militants, American and Iraqi special forces have regularly killed or captured several Mahdi army commanders. The drive against Sunni insurgents has increased as well, with Iraqi commandos reported to have killed 15 members of the Qaeda-linked Omar Brigade.