A suicide bomber rammed his car into a police patrol on the road to Baghdad's airport Wednesday, killing six people and wounding 12 others, police said.

Police said three officers were among those killed in the attack, while four policemen were wounded.

The heavily secured highway leading from central Baghdad to the capital's airport was once among the most dangerous stretches in Iraq. Insurgents staged almost daily attacks with roadside bombs and automatic weapons on U.S military convoys and civilian vehicles traveling to and from downtown Baghdad.

But security has improved markedly on the highway in the past 18 months since authorities blocked off side streets feeding onto the road put up blast walls along the highway.

Also Wednesday, two U.S. soldiers were wounded in a roadside bomb attack near the town of Karmah west of Baghdad as they responded to reports of a bomb in the area, the U.S. military said. Two Iraqis were killed, while an Iraqi interpreter, two Iraqi Army soldiers and 11 civilians were wounded in the bombing.

An Iraqi security official said the U.S. troops were injured as they rushed to the site of an earlier roadside bomb attack that killed a prominent leader of the local awakening council, the term for mostly Sunni groups that have joined forces with the Americans against al-Qaida in Iraq.

Elsewhere, one policeman died and three more were wounded when their patrol hit a roadside bomb in Amarah, 200 miles (320 kilometers) southeast of Baghdad, police said.

Wednesday's violence follows a series of bombings in Baghdad this week that have killed more than 30 people, and underscores that insurgents still pose a threat.

Meanwhile, the U.S. military said an American soldier died of non-combat related causes.

It was the first death reported since Oct. 29 among the 151,000-strong American military force in Iraq, and brought to at least 4,191 the number of U.S. military members who have died in the Iraq war since it began in March 2003, according to an Associated Press count.

U.S. officials say attacks in the Iraqi capital are averaging about four a day — down nearly 90 percent from levels of late 2006, when Shiite-Sunni fighting was at its high point and just before the U.S. troop surge that helped bring down violence in the capital. But U.S. commanders warn the security gains are reversible.