Baghdad, Iraq ? Iraqi insurgents struck Saturday in the volatile Sunni Triangle west of Baghdad, killing five U.S. soldiers in separate bombings and narrowly missing an American convoy with a blast that killed four Iraqis and wounded about 40 others north of the capital.

The bloody attacks occurred as U.N. security experts began to study the possible return of U.N. international staff to play a key role in Iraq’s transformation to democracy. The thud of distant explosions rumbled across the capital late Saturday, heightening the sense of insecurity that still prevails nine months after the collapse of Saddam Hussein’s regime.

In Khaldiyah, some 70 miles west of Baghdad, three U.S. soldiers were killed and six more were wounded when a vehicle, possibly driven by a suicide bomber, exploded at a U.S. checkpoint near a bridge across the Euphrates River, the U.S. command said.

Iraqi witnesses said a four-wheel-drive vehicle drove up to the checkpoint and exploded in front of a U.S. Army Humvee trying to block it. At least eight Iraqis — six of them women — were injured, according to Dr. Ahmed Nasrat Jabouri of the provincial hospital in nearby Ramadi.

“It shook the whole area,” Emad Ghareb Hamid said of the blast. U.S. troops sealed off the area while ambulances and helicopters evacuated the casualties.

Earlier Saturday, two other U.S. soldiers were killed by a roadside bomb that struck their four-vehicle convoy north of Fallujah, a Sunni Muslim city near Khaldiyah in a center of anti-American resistance.

The latest deaths brought to 512 the number of U.S. service members who have died since the United States and its allies launched the Iraq war March 20. Most of the deaths have occurred since President Bush declared an end to active combat May 1.

A third attack took place when a truck bomb exploded Saturday morning near government buildings in Samarra, about 70 miles north of Baghdad, barely missing a U.S. military police patrol as it turned into a police station compound.

The blast killed four Iraqi civilians and wounded about 40 people, including seven American soldiers who were cut by flying glass inside one of the buildings, according to Capt. Jennifer Knight of the 720th Military Police Battalion. The Americans’ wounds were not life-threatening.

The explosion set fire to a half-dozen cars parked near the buildings, which included a police station and municipal offices, and gouged a large crater in the street.

Resistance to the American occupation has persisted in the Sunni heartland north and west of Baghdad, despite the Dec. 13 capture of Saddam.

An Iraqi policeman guards the area where a truck bomb exploded in Samarra, Iraq, as a U.S. patrol passed by. Four Iraqi civilians were killed and about 40 people were injured in Saturday's blast, including seven U.S. soldiers. Two other bombings Saturday in Iraq killed five American soldiers.