BAGHDAD — The U.S. military has launched a major counter-insurgency operation near the Iraqi border with Syria.

The military said the offensive began on May 8 and so far at least 100 insurgents were killed in the western province of Al Anbar. Officials said the operation sought to eliminate insurgency strongholds and supply routes from Syria.

"Al Zarqawi has established a series of strongholds along the Syrian border," a U.S. military source said. "There, he keeps his fighters, supplies, weapons and bomb factories."

The focus of the operation was in Al Qaim in northwestern Iraq, Middle East Newsline reported.

Officials said Marine Regimental Combat Team-2, responsible for Al Anbar, struck with ground forces and attack helicopters. Officials said four Marines were killed, three of them on Monday.

Sunni insurgents fought with mortars, rocket-propelled grenades and semi-automatic fire, officials said. They said some of the insurgents were wearing flak jackets that appeared to have arrived from Syria.

Officials said that despite numerous warnings Syria has failed to block the flow of insurgents, funding and supplies to Iraq. They said the insurgents were linked to the upsurge in suicide strikes around Baghdad over the last few weeks.

"We have seen insurgents from such places as Jordan, the Palestinian areas, Saudi Arabia and Syria," a military official said.

The fresh weapons and insurgents were said to have bolstered the Al Qaida network in Iraq, led by Abu Mussib Al Zarqawi. On May 5, U.S. troops captured an associate of Al Zarqawi, identified as Amar Hamzah Zubaydi, said to have planned several suicide attacks as well as April's assault on Abu Ghraib prison.

On April 26, another Al Zarqawi aide, identified as Ghassan Al Rawi, was arrested. Al Rawi was responsible for coordinating meetings of the Al Qaida leadership, planning Al Zarqawi's activities and helping obtain bombs and steal cars in Rawah, near the Syrian border.

Officials said Zarqawi's network acquired scores of stolen cars and massive bombs, later used in the Al Qaida offensive against Shi'ite areas of Baghdad and other cities in April and May. About 300 people have been killed since April 28, and during that month a record 135 bombs were detonated in Iraq, nearly double that of the previous month.

The rash of suicide bombings came amid the capture or killing of more than 20 aides of Al Zarqawi in 2005. The military said at least 54 operatives of Al Zarqawi as well as car bombs and weapons were captured on May 8 in Al Qaida near the Syrian border.

Officials identified Al Zarqawi's chief aide as Abu Talha, who comes from the northern Iraqi city of Mosul and responsible for attacks that killed hundreds of Iraqi civilians. They said Al Zarqawi, nearly captured on Feb. 20, has obtained haven with local tribes.

"Although Zarqawi's network has been diminished, his followers can still muster forces for attacks," Multi-National Force-Iraq spokesman Col. Don Alston said. "While these attacks have targeted [Iraqi security forces], coalition forces and Iraqi citizens, they have resulted most often in killing numerous innocent Iraqi civilians."