Qaim, Iraq ? With snipers on rooftops and helicopters hovering overhead, U.S. forces clashed with insurgent fighters Monday 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 further 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.

Oil Minister Ibrahim Bahr al-Uloum was headed out of the capital to attend the opening of a rebuilt refinery to the north when the roadside bomb hit his seven-car motorcade Monday 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 on pipelines and refineries.

Suspected insurgents hold weapons in the streets of Ramadi, Iraq, Monday following reported clashes between gunmen and Iraqi security forces in the early morning hours.

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, codenamed Operation Iron Fist, began Saturday in the village of Sadah and has spread to Karabilah and Rumana.

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

The military said it confirmed at least 21 militants killed, two in fighting Monday 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 from indirect fire Saturday in Ramadi, the military said Monday. The death raised to 1,936 the number of U.S. military members who have died since the Iraq war began in 2003.