BAGHDAD, Iraq - A coordinated string of four bomb attacks within seven minutes killed at least 18 people and wounded 39 in northern Iraq today, while a Baghdad car bomb injured 28, officials said, ending a relative lull in violence that had fallen over the country in recent days.

Hundreds of U.S. and Iraqi soldiers descended on the remote northern city of Tal Afar near the Syrian border today, launching a major operation against insurgents following weeks of attacks against Iraqi security services there, military officials said.

Two U.S. Marines died Monday after separate roadside bombings near Fallujah, 40 miles west of Baghdad, the military said today.

Today's attacks in northern Iraq appeared coordinated and aimed at checkpoints manned by members of Iraq's fledgling army, which has been a constant target of insurgents opposed to the country's new U.S.-backed government.

The first explosion, caused by a roadside bomb, rocked Hawija, about 40 miles south of Kirkuk, at around 9:30 a.m. Soon after, three suicide bombers waiting in lines of cars at army checkpoints to the west and north of Hawija struck in quick succession.

In the deadliest attack, 10 civilians and one soldier were killed at a checkpoint in Dibis, two miles west of Hawija, army Lt. Faleh Ahmed said. Three soldiers and two civilians were killed at a checkpoint in Bagara, three miles west of Hawija. Two soldiers died in a suicide attack on the Aziziya checkpoint at the northern entrance to Hawija.

"I was standing some distance from the checkpoint when I heard a big explosion and I was thrown onto the ground," Lt. Sadiq Mohammed 26, whose right leg was wounded in the Dibis attack, said from his hospital bed. "This is a terrorist act because real resistance should only target American troops, not Iraqis trying to protect their country."

The Tal Afar offensive is targeting "terror suspects" responsible for multiple attacks on civilians, U.S. Army spokesman Sgt. John Franzen said.

Eyewitnesses said U.S. helicopters were flying overhead as tanks and Bradley fighting vehicles patrolled its narrow streets, regularly coming under small arms fire from militants. Tal Afar police Capt. Amjad Hashim said about 20 suspected insurgents had been captured.

Hashim also claimed insurgents fired an anti-aircraft rocket at a U.S. helicopter, striking it and making it return to its base. Franzen said an American OH-58 two seat reconnaissance helicopter suffered "mechanical difficulties" but was unaware if it had come under enemy attack.

The Tal Afar operation comes a day after U.S. and Iraqi military commanders met with nearly 80 local tribal elders and agreed to "work together to end violence" and rebuild the ancient city's police and local government services, the military said in a statement.

The two U.S. Marines died from wounds after roadside bombings Sunday and Monday near the volatile city of Fallujah, 40 miles west of Baghdad, the military said today. At least 1,672 U.S. military members have died since the Iraq war began in March 2003, according to an Associated Press count.

More than 860 people have died during the less than six-week period since Iraq's new Shiite-led government was announced.

But Iraqi and U.S. officials maintain that a high-profile counterinsurgency offensive in Baghdad, dubbed Operation Lightning, has helped curb the number of attacks in the capital, where multiple suicide car bombings and drive-by shootings have become a sinister part of daily life.

The operation, which began May 22, is the biggest Iraqi-led offensive since Saddam's ouster two years ago. Before it began, authorities controlled only eight of Baghdad's 23 entrances. Now all are under government control. At least 887 arrests have been made during the operation, according to government figures, and 608 mobile and 194 permanent checkpoints have been established around Baghdad.

Sunni Arab Islamic extremists opposed to the new Shiite-led government and former Saddam Hussein loyalists, who lost their positions of power following the former dictator's ouster, are believed to be major players in the rampant insurgency.

Saddam, who was captured in December 2003, is facing about 14 cases ranging from gassing thousands of Kurds and suppressing a Shiite uprising to executing religious and political foes during his 23-year reign, according to a list obtained by The Associated Press.

The man who once ruled Iraq with an iron fist will likely take the stand behind a bulletproof glass dock in a custom-made court room, reportedly being built inside Baghdad's heavily fortified Green Zone, the base of Iraq's government and home to the U.S. Embassy.

Elsewhere, the body of a Sunni cleric who was shot dead was found today underneath a bridge in the southern Iraqi city of Basra, a relative of the man and a local mosque leader said.

Salam al-Kardici, 50, was kidnapped Sunday by armed men wearing police uniforms, relative Abdullah Abdul-Karim said.