Clashes also resumed in Fallujah, a one-time insurgent stronghold that US forces believed they had conquered.
The violence underlines the difficulties US-led forces have encountered in the year-and-a-half since Saddam was ousted in trying to end a rampant insurgency and bring the country under control.
US military commanders acknowledge they initially underestimated the strength of the insurgent backlash and admit coalition-trained Iraqi security forces are not yet up to securing their own country.
The fighting in Anbar, a vast province including Fallujah and Ramadi, was the deadliest for US forces since eight marines were killed by a car bomb outside Fallujah on October 30.
The deaths brought to nearly 1,300 the number of US troops killed in Iraq since the US-led invasion in March 2003.
In Baghdad, a militant in an explosives-laden car detonated the vehicle as he drove toward a checkpoint at the western gate of the heavily fortified Green Zone, where the US Embassy and Iraq’s interim government is located, police said.
Dr Mohammed Abdel Satar of Baghdad’s Yarmouk Hospital said 13 people were killed and 15 wounded in the suicide blast.
An Islamic website carried a message saying al-Qaida ally Abu Musab al-Zarqawi’s group claimed responsibility.
The international zone has been the scene of frequent insurgent attacks in the past 18 months, killing and wounding dozens of people in car bombings or mortar barrages.
In Tarmiyah, north of Baghdad, a car bomb exploded and wrecked two US Humvees, wounding three US soldiers and an Iraqi civilian.
In Mishahda, 25 miles north of Baghdad, gunmen attacked an Iraqi National Guard patrol, killing three soldiers and wounding three others. The attackers fled, witnesses said.
Iraq’s interim President Ghazi al-Yawer said in an interview broadcast yesterday that the US-led coalition was wrong to dismantle the Iraqi security forces after last year’s invasion. “Definitely, dissolving the Ministry of Defence and the Ministry of Interior was a big mistake at that time,” Mr al-Yawer told the BBC.
It would have been more effective to screen out former regime loyalists than rebuild from scratch, he said.
In the central Iraqi city of Samarra, insurgents attacked patrolling US soldiers with small arms fire and rocket-propelled grenades. One detonated near a group of children, killing a nine-year-old boy.