HADITHA, Iraq – American 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 Qaeda insurgents. At least five American service members have been killed in the fighting.

Operation River Gate – launched at the start of the holy month of Ramadan – was the second American offensive in a week in Anbar province, near the Syrian border. Al Qaeda in Iraq called for intensified attacks on American and Iraqi forces during the Muslim period of fasting, which started yesterday for the nation’s Sunnis.

Blasts from American warplanes and helicopters lit up the sky during the fighting, aimed at putting down Sunniled insurgents intensifying their campaign of violence ahead of an October 15 vote on Iraq’s new constitution.

As with the earlier American offensive – code named Iraq Fist – it appeared many fighters may have slipped away beforehand.

On the political front, American and U.N. officials were trying to avert a Sunni Arab boycott of the referendum, which would deeply undermine the validity of a constitution Washington hopes will unite Iraq’s factions and weaken the insurgency.

Officials met yesterday with Shiite and Kurdish leaders to persuade them to reverse the Shiite-led government’s last-minute change to voting rules ahead of the October 15 vote. Sunday’s change makes it almost impossible for minority Sunnis to reject the constitution – all guaranteeing its passage.

Sunni Arab leaders have opposed the draft constitution all along, but America wants them to participate in the vote.

Late Monday and early yesterday, about 2,500 American troops along with Iraqi forces launched their operation with a powerful air assault on Haditha, Haqlaniyah, and Parwana, about 140 miles northwest of Baghdad.

Dozens of helicopters streamed toward the Euphrates towns in a phalanx, seen by an Associated Press reporter. Rockets fired by choppers flashed in the pre-dawn darkness, followed by explosions and arches of tracer fire.

At least twice, illuminating flares went up over Haqlaniyah – a sign American troops were fighting insurgents on the ground.

American warplanes struck bridges to prevent Al Qaeda 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 American service members, apparently among the first ground troops to move in.

American snipers took positions on rooftops in Haqlaniyah as troops with loudspeakers ordered residents to stay inside, witnesses said.

In Haditha, mosque loudspeakers urged residents to confront the Americans, but Marines said they encountered little resistance.