QAIM, Iraq — With snipers on rooftops and helicopters hovering overhead, U.S. forces clashed with insurgent fighters yesterday while searching homes in a town near the Syrian border.

In Baghdad, Iraq's oil minister narrowly escaped an assassination attempt when a bomb hit his motorcade.

While U.S. forces pushed ahead with their offensive farther west, fighting erupted in the capital of Iraq's Anbar province, with masked militants attacking an Iraqi patrol and sparking a gunbattle in the streets of Ramadi, 70 miles west of Baghdad.

Oil Minister Ibrahim Bahr al-Uloum was leaving Baghdad to attend the reopening of a refinery to the north when the roadside bomb hit his seven-car motorcade yesterday morning, killing three of his bodyguards, the ministry said. Bahr al-Uloum was unhurt.

The assassination attempt came a week after a car bomb at a checkpoint near the Oil Ministry killed at least three ministry employees and seven policemen.

Iraq has the world's third-largest known oil reserves, but the industry has been crippled by war, sanctions during Saddam Hussein's rule and the anti-U.S. insurgency. Oil production remains limited, curbed by decaying infrastructure and frequent militant attacks.

The violence came less than two weeks before a national referendum on a new constitution. Al-Qaida in Iraq and other groups in the Sunni-led insurgency have launched a wave of violence to wreck the Oct. 15 vote, killing at least 207 people in the past eight days, including 16 U.S. forces.

The U.S. offensive near the western border aims to sweep out al-Qaida insurgents in Iraq who have made the area a stronghold and used it to bring foreign fighters in from Syria.

The sweep began Saturday in the village of Sadah and has spread to Karabilah and Rumana on the banks of the Euphrates River, 180 miles northwest of Baghdad.

U.S. helicopters fired rockets at targets in Rumana, where a roadside bomb blew up near an American armored vehicle. No U.S. casualties were reported.

In Karabilah, troops searched house-to-house for militants, apparently meeting stiffer resistance than in Sadah, which most fighters fled before the U.S. moved in.

Marines fired from rooftops and U.S. helicopters flew overhead as the advance was slowed for an hour by insurgent fire, a CNN journalist embedded with the Marines said.

At one point, about 20 Iraqis fled their homes, including one family — a couple and their child — who were bleeding after being hit by flying pieces of concrete, CNN footage showed.

The military said it confirmed at least 21 militants killed, two in fighting yesterday and 19 from an airstrike the day before, bringing the three-day total to 57.

No U.S. troops have been killed or seriously injured in the offensive, the military said.

But an American soldier died of wounds suffered Saturday in Ramadi, the military said yesterday. The death raised to 1,936 the number of U.S. military members who have died since the Iraq war began in 2003.

The U.S. also dismissed as "patently false" a claim by al-Qaida in Iraq that its insurgents had captured two Marines in the fighting, even as the group issued a claim it had killed them.

In Ramadi, insurgents attacked an Iraqi army patrol, setting one vehicle on fire and sparking a gunbattle.

Gunmen in black hoods were seen with machine guns and rocket-propelled grenades in the streets, and Iraqi civilians gathered around the two burning Iraqi army pickups.

But the insurgents appeared to have taken the worst of the fight. Seven gunmen were killed, said Capt. Jeffrey Pool, a U.S. military spokesman. No casualties were reported among the Iraqi troops.