BAGHDAD, Iraq — The US military yesterday reported six more American troops killed in fighting the day before, raising the toll to 25 in the deadliest day for US forces in Iraq in two years.Four US soldiers and a Marine were killed Saturday during combat in Anbar, the Sunni insurgent stronghold west of Baghdad, the military said. A roadside bomb also struck a security patrol northeast of Baghdad, killing one soldier.

Saturday’s carnage also included 12 soldiers killed in a Blackhawk helicopter crash northeast of Baghdad, five killed in a militant attack in the holy Shiite city of Karbala and two others slain elsewhere in roadside bombings.

The parliamentary bloc loyal to the radical Shiite cleric Muqtada al-Sadr, meanwhile, ended its nearly two-month 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 US forces.

“We announce our return to parliament, we will attend today’s session, and the ministers will resume their work to serve the people,” Bahaa al-Araji, one of 30 lawmakers loyal to al-Sadr, said during a news conference attended by Sunni parliament speaker Mahmoud al-Mashhadani. Al-Sadr also has six loyalist ministers in the 38-member Cabinet.

The decision appeared to be a way for both sides to save face while allowing al-Sadr’s bloc, whose support is crucial to Shiite Prime Minister Nouri al-Maliki, to regain legislative influence ahead of a planned US-Iraqi security crackdown in Baghdad.

“We are on the verge of a new era. We will be victorious and we will achieve the major change through unity and fraternity and be our army’s regain of its strength, taking responsibility and fighting terrorism,” al-Maliki said separately during a ceremony for military academy students.

The first reinforcements of US troops have already started to flow into the region. A brigade of the US 82nd Airborne Division, part of the buildup, has arrived in Baghdad and its 3,200 soldiers will be ready to join the fresh drive to quell sectarian violence in the capital by the first of the month, the American military said yesterday.

But the deadly toll among US forces comes at a critical time of rising congressional opposition to President Bush’s decision to dispatch 21,500 additional soldiers to the conflict.

The US military statement about the Karbala attack said “an illegally armed militia group” attacked the provincial headquarters building with grenades, small arms and “indirect fire,” which usually means mortars or rockets.

“A meeting was taking place at the time of the attack to ensure the security of Shiite pilgrims participating in the Ashoura commemorations,” said a statement from Brig. Gen. Vincent K. Brooks, deputy commander of the Multi-National Division-Baghdad.

Thousands of Shiite pilgrims are flocking to the city to mark the 10-day Ashoura festival commemorating the death of one of Shiite Islam’s most sacred saints, Imam Hussein, grandson of the Prophet Muhammad.

Provincial Gov. Akeel al-Khazaali, who was not at the security meeting, said the gunmen, dressed in military uniforms, were able to drive their black SUVs — similar to those driven by foreign dignitaries — through a checkpoint on the outskirts of the city, 50 miles south of Baghdad, because police assumed it was a diplomatic convoy and informed headquarters that it was coming.

“The group used percussion bombs and broke into the building, killed five Americans and kidnapped two others, then fled,” the governor said, adding that Iraqi troops later found one of the SUVs with three bodies of uniformed men.

The US military, which has said that five US soldiers were killed and three were wounded while repelling the attack, denied that two US troops were kidnapped.

Lt. Col. Christopher Garver, a military spokesman, said all American forces “were accounted for after the action.”

A security official in Karbala, speaking on condition of anonymity because he was not authorised to disclose the information to the media, said the gunmen drove to Babil province after the attack. The Babil police commander confirmed that they entered the region before disappearing.

Although Babil province is predominantly Shiite, some parts of it, just south of Baghdad, are Sunni and insurgents are known to be active there.

Saturday was the deadliest day for US forces in two years. It was also the third-highest of any single day since the war began in March 2003, eclipsed only by 37 US deaths on January 26, 2005, and 28 on the third day of the US invasion. US authorities also announced two American combat deaths from Friday.

In violence reported by police yesterday:

[bul] A bomb left in a bag struck a small bus carrying people to work in a predominantly Shiite area in Baghdad, killing seven passengers and wounding 15.

[bul] A parked car bomb also exploded outside a restaurant in eastern Baghdad, killing one person and wounding five, according to police.

[bul] A suicide car bomber targeting an Iraqi army patrol killed one woman and wounded five other people in the northern city of Mosul.