BAGHDAD, Iraq (CNN) -- Five U.S. soldiers and four Iraqi civilians were killed Saturday in three separate bomb attacks in Iraq's so-called Sunni Triangle, U.S. military officials said.

In Khaldiyah on Saturday, a car bomb killed three U.S. soldiers and wounded six others. Several Iraqi civilians also were wounded in the attack. Two of the wounded soldiers were taken to a combat support hospital and four were being treated at a local military base.

West of Fallujah on Saturday a roadside bomb exploded and killed two U.S. soldiers as their vehicle passed by. The soldiers were assigned to the U.S. Army's 82nd Airborne Division, U.S. Central Command said. No more details were immediately available.

In another Saturday attack, a bomb hidden along a road near a government building in the northern Iraqi city of Samarra exploded as a U.S. military convoy passed by. The blast killed at least four Iraqi civilians and seriously wounded another. Thirty-three civilians were treated for minor wounds at a local hospital. Three U.S. soldiers also were wounded, U.S. military sources said.

The bomb was probably detonated remotely and was hidden near a Toyota pickup truck parked near the building, the sources said.

Insurgents have been active all this week in the volatile Sunni Triangle -- where three attacks in a 24-hour period Wednesday and Thursday killed nine people, including two U.S. soldiers.

The latest deaths bring the number of U.S. soldiers killed in the war to 511.

In recent days, U.S. commanders in Iraq expressed fears of al Qaeda attempting to link up with the anti-U.S. insurgency.

The violence comes as U.S. officials announced the arrests in Iraq of two men linked to al Qaeda: Hasan Ghul, a senior operative with the terrorist network, and Husam al Yemeni, a top lieutenant to a man operating with Ansar al-Islam, a group linked to al Qaeda.

U.S. officials: Al Qaeda agent arrested in Iraq

Pro-coalition forces captured Ghul in Iraq and turned him over to U.S. intelligence personnel, senior U.S. officials said Friday.

A U.S. official said Ghul is a "longtime facilitator, operator" within al Qaeda, and a "significant player."

Earlier Friday, Pentagon sources announced that U.S. forces captured Yemeni last week during a raid near Fallujah. He is said to be a key associate of Abu Musab al-Zarqawi. Pentagon officials call the capture "significant" and say it suggests they may be getting closer to finding Zarqawi.

Meanwhile, senior U.S. military officials said U.S.-led coalition forces are making progress in their hunt for Izzat Ibrahim al-Douri, the highest-level Saddam Hussein loyalist still on the run.

Senior coalition sources have said they believe Ibrahim is responsible for organizing resistance fighters in Baghdad, Fallujah, Ramadi, Samarra and Tikrit.

Iraqi insurgents take on al Qaeda tactics

The commander of U.S. ground forces in Iraq said he believes that insurgents in Iraq, whose tactics resemble those of al Qaeda, may be receiving financial support from the terror group.

Lt. Gen. Ricardo Sanchez declined to provide specific evidence that rebel forces in Iraq were directly linked to Osama bin Laden's terrorist network.

"I think it's probably not appropriate for me to talk about al Qaeda in the sense of a concrete, proven presence," Sanchez said. "We're seeing al Qaeda-like tactics. We believe that there's training that's been conducted for some of the terrorists."

Although the instruction is not happening in Iraq, he said, al Qaeda seems to be training "those elements that are operating in here. And we think that there's also financing that has been taking place."