In Kirkuk

IN KIRKUK: Smoke rises after a car bombing, one of two blasts outside churches in the northern Iraqi city on the same day. No one was reported injured. Days before, churches in Mosul were targeted. (AP)

U.S. bombers and fighter jets continued an aggressive attack on the southern outskirts of Baghdad this morning, unleashing 38 bombs in 10 minutes on suspected Al Qaeda in Iraq safe havens.

In all, they dropped 40,000 pounds of bombs on Arab Jabour, in an area of mostly farmland, the U.S. military said in a statement.

Like Diyala, Arab Jabour has been plagued with ongoing violence, despite efforts to purge Al Qaeda. Some Awakening Councils — U.S.-backed security forces of former Sunni insurgents — have formed nearby, but none have achieved much success in the district on the outskirts of the capital.

Senior Sunni Arab insurgents may have fled the Diyala River valley this week just as U.S. troops were preparing to attack, but they left behind a deadly calling card.

A booby-trapped home exploded Wednesday, killing six American soldiers and injuring four others. The U.S. military also reported that three service members were killed by small-arms fire the day before. The two-day toll makes the latest effort to flush out the militant group Al Qaeda in Iraq the deadliest military operation in months.

The casualties came as about 4,000 U.S. and Iraqi troops descended Tuesday on Diyala province as part of a campaign to put new pressure on insurgents nationwide. Military officials believe many settled in the area north of Baghdad after being forced out of the capital and Anbar province in the west.

At least 3,921 U.S. troops have been killed since the March 2003 invasion of Iraq, according to the independent website icasualties.org. The last time six American troops were killed in a single hostile incident was in late May, in a roadside bombing in the Diyala community of Abu Sayda.

The Diyala region accounts for more than 40% of attacks nationwide. Intelligence reports estimated that 50 to 60 senior insurgent leaders had been holed up northwest of Muqdadiya, but by the time the offensive began, they had fled -- in keeping with a long-standing pattern.

As U.S. forces continue to press into areas where they have not regularly patrolled, they have been at greater risk of encountering homes rigged with large amounts of explosives, officials said.

The military offered no details about Wednesday's deadly attack, nor did it release the names of the dead soldiers, pending notification of their families.

"We are looking really closely at the tactic," said Edward Loomis, a spokesman for the Pentagon's Joint Improvised Explosive Device Defeat Organization. "We will continue to do everything we can to lower the risk of these events occurring. We are going to look really hard at this one."

Rigged houses typically use explosives and triggering devices similar to those in roadside bombs or car bombs, which the military calls vehicle-borne IEDs.

U.S. forces in Iraq first encountered large numbers of booby-trapped houses during the battle of Fallouja in 2004. American forces had steered clear of the city in Anbar province for much of that year, then telegraphed their intention to clear the city of Sunni Arab insurgents weeks before the operation began, allowing them to prepare elaborate defenses.

A number of rigged homes were also found in Diyala province in May and June, Loomis said, as U.S. forces stepped up operations against Sunni insurgents. During the previous Diyala operations some military officers referred to such homes as house-borne IEDs.

In the past, when such homes were discovered before they detonated, Air Force fighter planes were used to destroy them.

On Sunday, soldiers south of Baqubah spotted suspected insurgents with grenade launchers and assault rifles unrolling wire around a building. U.S.-led forces launched a Hellfire missile at the building, then dropped two bombs on it.

Secondary blasts, and wire discovered at the site, confirmed that the building had been rigged with explosives, U.S. military officials said.

On Wednesday, U.S. and Iraqi forces combed isolated villages, dense orchards and palm groves.

Mortar rounds crashed through thick foliage ahead of the advance through the agriculturally rich area, known as the breadbasket of Iraq.