The U.S. military reported six more American troops killed in fighting the day before, raising the death toll to 25 — the third highest of the war. Muqtada al-Sadr's bloc, meanwhile, announced it was returning to the political fold in an apparent bid to regain legislative influence ahead of a planned security crackdown.

The Sadrists said they were lifting their 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 U.S. 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.

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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 U.S.-Iraqi security crackdown in Baghdad.

The current year will witness major changes in political stability, economy and public services to people, al-Maliki said Sunday during a ceremony for military academy students.

"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.

The first reinforcements of U.S. troops under the new Bush strategy already have started to flow into the Baghdad region. A 3,200 member brigade of the U.S. 82nd Airborne Division, part of the buildup, has arrived in Baghdad and 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 Sunday.

The 2nd Brigade of the 82nd Airborne consists of about 3,200 soldiers who will "assist Iraqi Security Forces to clear, control and retain key areas of the capital city in order to reduce violence and to set the conditions for a transition to full Iraqi control of security in the city," the military said in a statement.

It will be fully operational within Baghdad on or about Feb. 1, the statement said.

The military said four U.S. soldiers and a Marine were killed in combat Saturday in Anbar province, the Sunni insurgent stronghold west of Baghdad, and Multi-National Division Baghdad soldier was killed by a roadside bomb in the capital, raising that day's toll among American forces to at least 25.

Iraqi officials, meanwhile, said the gunmen who attacked the provincial headquarters in the Shiite holy city of Karbala, killing five other U.S. troops on Saturday, were wearing military uniforms and drove up in black sport utility vehicles commonly used by foreign dignitaries — an apparent attempt to impersonate Americans.

The local governor said the gunmen stormed into the building during a U.S.-Iraqi meeting to discuss security measures ahead of the Shiite Ashoura festival.

In the Karbala incident, provincial Gov. Akeel al-Khazaali, who was not at the security meeting, said the SUVs were able to get 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 to the area near Mussayib," about 20 kilometers to the north, the governor said, adding that Iraqi troops later found one of the SUVs with the three dead bodies dressed in military uniforms.

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

"Nothing has changed since the night before," Lt. Col. Christopher Garver said. "During the attack on our coalition forces, we sustained five U.S. KIA and three US wounded. All of MND-Baghdad [multinational division] personnel were accounted for after the action."

A security official in Karbala, speaking on condition of anonymity because he was not authorized to disclose the information to the media, also said the gunmen who carried out the attack on the Provincial Joint Coordination Center were using SUVs similar to ones used by the U.S. authorities. He said during their attack, the gunmen used stun grenades then left shortly afterward.

The official said the convoy of gunmen then into Babil province. The police commander in the province 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.

The deaths of the U.S. troops, combined with a helicopter crash that killed 12 U.S. soldiers, made Saturday the deadliest day for U.S. 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 U.S. deaths on Jan. 26, 2005, and 28 on the third day of the U.S. invasion. U.S. authorities also announced two American combat deaths from Friday.

The heavy toll comes at a time of rising congressional opposition to U.S. President George W. Bush's decision to dispatch 21,500 additional soldiers to the conflict to try to curb sectarian slaughter.

The military gave little information on the crash of the Army Black Hawk helicopter during good weather in Diyala province, northeast of Baghdad. U.S. and Iraqi forces have been battling Sunni insurgents and Shiite militias for months in the province, around the city of Baqouba.

Lt. Col. Josslyn Aberle, a U.S. spokeswoman, said the cause of the crash had not been determined. Navy Capt. Frank Pascual, a member of a U.S. media relations team in the United Arab Emirates, told Al-Arabiya television that the helicopter was believed to have suffered technical troubles before going down.

The military initially said 13 people were on board the aircraft but corrected the number on Sunday, saying 12 soldiers died, including eight passengers and four crew members.

Also Saturday, roadside bombs killed a soldier in the capital and one in Nineveh province north of Baghdad.

The U.S. 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.

Brooks said Iraqi officials and security forces as well as U.S. troops were present at the meeting, but his statement did not mention other casualties from the attack. It said the headquarters had "been secured by coalition and Iraqi security forces."

In violence reported by police on Sunday:

— 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.

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

— A homicide car bomber targeting an Iraqi army patrol killed one woman and wounded five other people in the northern city of Mosul.

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