HADITHA, Iraq — U.S. troops pushed through streets sown with bombs yesterday in their biggest operation this year in western Iraq, seeking to retake three Euphrates River towns from al-Qaida insurgents. At least five U.S. service members have been killed in the fighting.

The deaths raised to at least 1,941 the number of U.S. military members who have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.

The second U.S. offensive in a week was launched in Anbar province, near the Syrian border, at the start of the holy month of Ramadan. The group calling itself Al-Qaida in Iraq called for intensified attacks on U.S. and Iraqi forces during the Muslim period of fasting.

U.S. warplanes and helicopters led the operation aimed at putting down Sunni-led insurgents intensifying their campaign of violence ahead of an Oct. 15 vote on Iraq's new constitution.

Late Monday and early yesterday, some 2,500 U.S. troops along with Iraqi forces launched their operation with a powerful air assault on Haditha, Haqlaniyah and Parwana, about 140 miles northwest of Baghdad. It was the strongest participation this year by Iraqi troops — U.S. commanders said hundreds were involved — at a time of deep concerns about their readiness.

American warplanes struck bridges to prevent al-Qaida in Iraq fighters from escaping the towns, but arriving troops encountered dozens of roadside bombs on main avenues, apparently in anticipation of the operation, Marine commanders said.

A single roadside bomb in Haqlaniyah on Monday killed three U.S. service members, apparently among the first ground troops to move in.

The earlier U.S. offensive began Saturday, 93 miles upriver by the Syrian border, and continued yesterday in the towns of Sadah, Karabilah and Rumana. A bomb killed a Marine in Karabilah, the first casualty of that operation.

The small, isolated towns near the border have been used as way stations for foreign fighters joining the fight. Haditha is also an "an important crossroads" for al-Qaida smuggling of fighters and weapons, the military said.