An al-Qaida-linked group posted a Web statement Tuesday claiming responsibility for a suicide car bombing that killed nine U.S. paratroopers and wounded 20 in the worst attack on American ground forces in Iraq in more than a year.

The Islamic State of Iraq, an umbrella group of Sunni militants that includes al-Qaida in Iraq, said it was behind Monday’s attack on a U.S. patrol base in Diyala province northeast of Baghdad, an area that has seen a spike in violence since American troops surged into the capital to halt violence there.

The victims were all members of the Army’s 82nd Airborne Division, said a spokesman for the Fort Bragg, N.C.-based unit. It was the highest number of casualties for the division since the war began, Maj. Tom Earnhardt said.

Meanwhile, police in the same area said gunmen disguised as Iraqi soldiers killed six Iraqis and burned five homes Tuesday in an unrelated attack. South of the capital, a family of seven was killed en masse, shot to death in their beds at dawn by masked gunmen, neighbors and police said.

British forces transferred another military base to Iraqi troops in the country’s south, ahead of the planned withdrawal this summer of about half of Britain’s contribution to the U.S.-led coalition here.

In Baghdad, two bombs went off outside the Iranian Embassy on Tuesday for the second consecutive day. Six civilians were injured, police said. Tension has risen over allegations by the U.S. and some Sunni politicians in Iraq about alleged Iranian interference in the country.

A suicide car bomber rammed into an American patrol base in Diyala on Monday, killing nine soldiers and wounding 20, the military said. All were members of the 82nd Airborne’s 5th Squadron, 73rd Cavalry Regiment, 3rd Brigade Combat Team, the military said. Fifteen of the 20 were treated and later returned to duty, while five others and an Iraqi civilian were evacuated to a medical facility, it said.

In its Web posting Tuesday, the Islamic State of Iraq put the number of Americans killed at 30.

“Almighty God has guided the soldiers of the Islamic State of Iraq to new methods of explosions,” it said without elaborating. The message appeared on a Web site that frequently airs communications from militants, but its authenticity could not be independently confirmed.

It was single deadliest attack on ground forces since Dec. 1, 2005, when a roadside bomb killed 10 Marines and wounded 11 on a foot patrol near Fallujah.

Twelve soldiers died when a Black Hawk helicopter crashed in Diyala on Jan. 20. The military said it might have been shot down but the investigation is still ongoing.

In other devastating attacks, 14 Marines were killed when a roadside bomb struck an amphibious assault vehicle near the western town of Haditha on Aug. 3, 2005, and a suicide bomber struck a mess tent in a base near Mosul on Dec. 21, 2004, killing 22 people, including 14 U.S. soldiers and three American contractors.

It was the second bold attack against a U.S. base north of Baghdad in just over two months and was notable for its use of a suicide car bomber. Militants have mostly used hit-and-run ambushes, roadside bombs or mortars on U.S. troops and stayed away from direct assaults on fortified military compounds to avoid U.S. firepower.

On Feb. 19, insurgents struck a U.S. combat post in Tarmiyah, about 30 miles north of Baghdad, killing two soldiers and wounding 17 in what the military called a “coordinated attack.” It began with a suicide car bombing followed by gunfire on soldiers pinned down in a former Iraqi police station where fuel storage tanks were set ablaze by the blast.

American troops are facing increasing danger as they step up their presence in outposts and police stations in Baghdad and areas surrounding the city, as part of the security crackdown to which President Bush has committed an extra 30,000 troops.

Sunni militants are believed to have withdrawn to surrounding areas such as Diyala where they have safe haven. The U.S. command also deployed an extra 700 soldiers to the province last month.

Another U.S. soldier was also killed Monday in a roadside bombing in Diyala, the military said — bringing the daily American death toll to 10. A British soldier was also shot to death while on patrol in the southern city of Basra, officials said.

The deaths raised to 85 the number of U.S. service members who died have in Iraq in April, making it the deadliest month for American troops since December, when 112 died.

On Tuesday, gunmen stormed a house south of Baghdad at dawn, going room to room and killing seven relatives while they were still in their beds, police and neighbors said.

Neighbors said they could not recognize the assailants, who wore masks and drove into town in three pickup trucks. The whole incident lasted just a few minutes, police said, with the gunmen methodically moving room to room and pumping bullets into the residents while they slept.

All seven victims were from the same family: a mother and father, their son and four teenage grandchildren, police said. The attack occurred in the mostly Shiite village of Jaara, less than 25 miles south of Baghdad.

Hours later, gunmen disguised as Iraqi soldiers raided a remote village near the city of Baqouba, killing six people, wounding 15, and burning five homes, police said.

The attack by about 70 gunmen riding in Iraqi army Humvees took place in the same province where the nine U.S. troops died a day earlier.

A brief transfer ceremony was held in Basra on Tuesday at the Shaibah logistics base, once the main center of British military operations in Iraq. Iraq’s national army planned to use the facility for training.

Two other British bases — al-Saie and Shatt al-Arab — were turned over to Iraqi forces in Basra, Iraq’s second-largest city, in the past month.

The bulk of Britain’s about 7,500 soldiers in the city will now operate from a base at Basra’s main airport.

Two car bombs exploded within minutes of each other Tuesday in a public parking lot about 150 yards from the front of the Iranian Embassy, wounding six civilians but causing no damage to the building, a police officer said on condition of anonymity out of concern for his own security.

On Monday, two parked car bombs exploded outside the embassy. One bomb exploded near the same public parking lot, killing one civilian and wounding another; the other parked car bomb exploded close to a police patrol near the embassy, killing one civilian and wounding two officers, police said.

On its Web site, the prominent Iraqi Sunni insurgent group Islamic Ansar al-Sunnah claimed responsibility for Monday’s bombing near the parking lot.

Also Tuesday:

— Four bullet-riddled bodies were delivered to the morgue in Kut, 100 miles southeast of Baghdad, said morgue official Mamoun Ajeel al-Rubaie. Three had been decapitated, he said.

— A bomb hidden in a bag exploded on a minibus in eastern Baghdad, killing four passengers and wounding eight, police said.

— U.S. forces captured 10 suspected insurgents and seized a cache of weapons in raids across central Iraq, the military said.