BAGHDAD, Iraq - U.S. forces backed by helicopter gunships and warplanes swept through a large area of western Iraq near the Syrian border for a third day today, raiding desert outposts and safe houses belonging to insurgents, the U.S. military said.

In Baghdad, two car bombs exploded during morning rush hour today, killing as many as seven and wounding 19, including three American soldiers, officials said.

As many as 100 militants have been killed since Operation Matador, the largest American military offensive in Iraq in six months, began Saturday night in the border town of Qaim, 200 miles west of Baghdad, the military said.

At least three U.S. Marines have been killed in the offensive, which was hunting for followers of Iraq's most wanted terrorist, Abu Musab al-Zarqawi, leader of Al-Qaida in Iraq, said U.S. officials.

A fourth U.S. Marine died Monday, but it was not immediately clear if that happened during the offensive, the military said.

A Los Angeles Times reporter embedded with the offensive said 20 U.S. troops also were wounded, but the U.S. military could not immediately confirm that.

Today, fighting was reported in Obeidi, 185 miles west of Baghdad, and the two nearby towns of Rommana and Karabilah, an Associated Press reporter in the region said. He said large numbers of Qaim residents were fleeing the area.

On Monday night, insurgents attempted to launch a counterattack 5 miles from U.S. Camp Gannon in Qaim, said U.S. Marine Capt. Jeffrey Pool. They attacked a Marine convoy with small arms fire, rocket-propelled grenades, roadside bombs and two suicide car bombers, Pool said in a statement.

The offensive by more than 1,000 Marines, sailors and soldiers included helicopter gunships, fighter jets, tanks and light armored vehicles. U.S. officials described the area as a known smuggling route and a haven for foreign fighters involved in Iraq's insurgency.

The New York Times reported today that Air Force F-15E Strike Eagle fighters dropped two 500-pound laser-guided bombs and fired 510 20-millimeter cannon rounds Sunday against insurgents around Qaim and that Marine F/A-18 fighters fired 319 20-millimeter cannon rounds.

The paper quoted U.S. Col. Bob Chase, chief of operations for the Second Marine Division, as saying: "The enemy honestly felt that they had a sense of security up there. It had been a safe haven, and a lot of folks up there were former Baathists," referring to Saddam Hussein's former ruling party.

"Now it is no longer a safe haven, and it will never be a safe haven again," said Chase. He was quoted as saying insurgents have had a network of illegal "rat lines" of men and materials moving from Syria into Iraq that had to be stopped, and said the offensive would continue for several days.