BAGHDAD, Iraq - A suicide car bomber linked to al-Qaida killed 13 people in Baghdad today, the first anniversary of Saddam Hussein's capture, and clashes resumed in Fallujah, a one-time insurgent stronghold that American forces believed they had conquered. Seven U.S. Marines died in combat in western Iraq.

The violence underlines the difficulties U.S.-led forces have encountered in the year since Saddam's ouster in trying to end a rampant insurgency. U.S. military commanders acknowledge they initially underestimated the strength of the insurgent backlash and say Iraqi security forces are not yet up to secure the country.

The fighting in Anbar, a vast province including Fallujah and Ramadi, was the deadliest for U.S. forces since eight Marines were killed by a car bomb outside Fallujah on Oct. 30. The deaths brought to nearly 1,300 the number of American troops killed in Iraq since the U.S.-led invasion in March 2003.

In Baghdad, a militant in an explosives-laden car waiting in line to enter the western Harthiyah gate of the heavily fortified Green Zone, which houses the U.S. Embassy and Iraq's interim government, detonated the vehicle as he drove toward the checkpoint, Iraqi police said.

Dr. Mohammed Abdel Satar of Baghdad's Yarmouk said 13 people were killed and at least 15 wounded in the suicide blast. The U.S. military said there were no injuries among its troops.

Jordanian militant Abu Musab al-Zarqawi's al-Qaida in Iraq group claimed responsibility for the bombing in a statement posted on an Islamic Internet site regularly used by militants.

"On this blessed day a lion from the (group's) Martyrs' Brigade has gone out to strike at a gathering of apostates and Americans in the Green Zone," the group said in a statement posted on an Islamic Web site.

The international zone has been the scene of frequent attacks by insurgents during the past 18 months, killing and wounding dozens of people in car bombings or mortar barrages.

Shortly afterward, three explosions were heard in central Baghdad, but it was unclear whether any damages or casualties were caused. U.S. forces were investigating.

In the town of Mishahda, 25 miles north of Baghdad gunmen attacked an Iraqi National Guard patrol killing three soldiers and wounding thee others. The attackers fled, witnesses said.

Iraq's interim President Ghazi al-Yawer said in an interview broadcast today that the U.S.-led coalition was wrong to dismantle the Iraqi security forces.

"Definitely dissolving the Ministry of Defense and the Ministry of Interior was a big mistake at that time," al-Yawer told British Broadcasting Corp. radio.

It would have been more effective to screen out former regime loyalists than to rebuild from scratch, he added.

"As soon as we have efficient security forces that we can depend on we can see the beginning of the withdrawal of forces from our friends and partners and I think it doesn't take years, it will take months," he said.

U.S. forces retook Fallujah from the insurgents in a bloody battle last month in which hundreds died, including at least 54 Americans. The city had fallen under the rule of radical clerics and their mujahedeen fighters after Marines lifted a three-week siege of the city in April.

At the time, U.S. commanders claimed the action had broken the back of the insurgency in the mainly Sunni Muslim areas of western Iraq and that Iraqi security forces would start being phased in to take over, but fighting in the region has continued.

"We have come light years from April when they (Iraqi security forces) refused to even come out to Fallujah," Marines Lt. Col. Dan Wilson said. "We are in the process of phasing more ISF into Fallujah ... (and) are better equipped to intuitively know who belongs in the city, and who does not."

On Sunday, American jets dropped 10 precision-guided missiles on rebel positions in Fallujah after insurgents fought running battles with coalition forces. It was unclear if there were any insurgent casualties.

"We are still running into some of these die-hard insurgents that have either come back into the city or have been laying low," spokesman Lt. Lyle Gilbert said. "As we are bringing in contractors to help with the reconstruction of Fallujah, this (fighting) slows the process down."

It was unclear whether the latest Marine deaths were connected with those clashes. The military said only that seven Marines died in two incidents while conducting "security and stabilization operations" in Anbar province.