BAGHDAD, Iraq - Two U.S. Marines were killed and 14 wounded when an explosive device hit their troop transport vehicle during a large American offensive being fought against suspected insurgents in western Iraq, the military said today. In Baghdad, militants detonated a car bomb in a market, killing six people, police said.

During the fifth day of Operation Matador near the Syrian border, hundreds of American troops in tanks and light armored vehicles continued to roll through desert outposts in search of followers of Abu Musab al-Zarqawi, leader of the al-Qaida in Iraq terrorist group, the U.S. military said.

Residents in the villages of Karabilah and Saadah reported heavy bombardments by U.S. artillery or warplanes today following what they believed was fighting in the area.

On Wednesday night, a U.S. Assault Amphibian Vehicle struck an explosive device outside the town of Husaybah, killing two Marines and wounding 14, American military spokesman Capt. Jeffrey Pool said today.

Earlier in the offensive, which began late Saturday night, at least three Marines were reported killed and 20 wounded. It is one of the biggest U.S. military operations in Iraq since Fallujah was taken from militants six months ago.

As many as 100 insurgents were killed in the first 48 hours of the offensive when U.S. forces clashed with well-organized and well-equipped fighters in Obeidi, 200 miles west of Baghdad, the U.S. military said.

Meanwhile, attacks by insurgents continued in other areas of Iraq, following a wave of bombs and gunfire that killed at least 69 people and wounded 160 Wednesday - pushing the death toll from insurgent violence to more than 400 in less than two weeks.

Today insurgents continued their attacks.

A car bomb exploded near a market in eastern Baghdad, killing at least six people and wounding 13, said police 1st Lt. Mazin Saeed. The blast also set some shops on fire in the New Baghdad area of the capital and destroyed 10 cars parked nearby, he said.

Elsewhere in Baghdad, suspected insurgents shot and killed Brig. Gen. Iyad Imad Mahdi as he drove to work at the Ministry of Defense. Col. Fadhil Muhammed Mobarak was shot and killed as he traveled to the Interior Ministry, where he led its police control room, police said.

Two car bombs also exploded in the northern city of Kirkuk, 180 miles north of Baghdad, police said. One blast occurred near a Shiite mosque, killing two people and wounding two, said police Capt. Sarhad Talabani.

The other exploded at a site where explosives experts were dismantling a roadside bomb that residents had found, said police Brig. Gen. Sarhad Qader. Two of the experts were wounded by the blast, which also destroyed nearby vehicles, Qader said.

The latest violence underscored how intense the fight for Iraq's future has become since the United States declared the end of major combat two years ago and since Iraqis voted in the country's first democratic elections in January.

Insurgents averaged about 70 attacks a day at the start of May, up from 30-40 in February and March, said Lt. Col. Steven Boylan, a spokesman for U.S. forces in Iraq.

Operation Matador was launched after U.S. intelligence showed insurgents had moved into the northern Jazirah Desert after losses in the cities of Fallujah and Ramadi.

Pool said Wednesday that the region is used as staging area for foreign fighters who cross into Iraq from Syria along ancient smuggling routes known as "rat lines."