HADITHA, Iraq (AP) -Rockets fired from U.S. helicopters lit up the sky. Then came the flash of explosions and tracer fire illuminating the target below – a cluster of Euphrates River towns ruled for months by al-Qaida insurgents.

Operation River Gate, launched early Tuesday with a powerful air assault, marked the U.S. military’s second major offensive in Anbar province in a week and its biggest this year in Iraq’s turbulent west.

The goal: to take back river towns near the Syrian border and put down insurgents intensifying their campaign of violence ahead of an Oct. 15 vote on Iraq’s new constitution.

As with the earlier U.S. offensive – code named Iraq Fist – there were signs that many fighters may have slipped away before U.S. and Iraqi forces arrived.

In Baghdad, on the political front, U.S. 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 Tuesday with Shiite and Kurdish leaders to persuade them to reverse the Shiite-led government’s last-minute change to voting rules ahead of the Oct. 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 the United States wants them to participate in the vote.

Tuesday also marked the start of the Muslim fasting month of Ramadan for Iraq’s Sunnis. The Sunni-led Al-Qaida in Iraq called for its fighters to turn Ramadan into a “month of defeat” for U.S. and Iraqi forces with intensified attacks.

Previous Ramadans saw spike in violence in Iraq – especially suicide attacks, in part because some Islamic extremists believe those who die in combat during Ramadan are especially blessed.

Some 2,500 U.S. troops along with Iraqi forces participated in Operation River Gate, targeting 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. At least twice, illuminating flares went up over Haqlaniyah – a sign U.S. troops were fighting insurgents on the ground.

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.

U.S. 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.

“Some of them may have gotten away. But those that are here, we’ll get them,” said Capt. Shannon Neller, of New York, commander of Lima Company in 3rd Battalion, 1st Marine Regiment.

Troops were concentrating on “systematically clearing” Haditha by doing house-to-house searches and locating bombs planted around town, Neller said.

“A lot of the locals we met were very helpful, and a lot pointed out insurgents or those who had been helping them,” Neller said. At least 11 people were detained.

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

At least 41 insurgents have been killed in Iraqi Fist, the U.S. military said. But many fighters appeared to have fled before the assault, with the military reporting no major engagements Tuesday.

The deaths in the two operations, along with that of a soldier shot in the western town of Taqaddum, raised to at least 1,940 the number of U.S. service members who have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.

Operation River Gate was notable for the strongest participation this year by Iraqi troops – U.S. commanders said hundreds were involved – at a time of deep concerns about their readiness.

The simultaneous strikes appeared aimed at breaking two insurgent strong points.

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.

Together, the Haditha area towns form a larger population center – with more than 100,000 residents – where militants have been operating almost freely after driving out Iraqi security forces with a series of bloody attacks earlier this year.

Twenty Marines and an Iraqi interpreter were killed in Haditha in an ambush and roadside bomb on Aug. 1 and Aug 3. Last spring, a suicide car bombing wrecked Haditha General Hospital, the region’s largest. The U.S. military has said that Iraq’s most-wanted terrorist, Abu Musab al-Zarqawi, head of Al-Qaida in Iraq, once had a home in Haditha.

The new U.S. assault is “step forward to eliminating insurgents and giving the country back to the Iraqi people,” said Col. Stephen W. Davis, who said it would also help people in area freely vote Oct. 15.

Insurgent violence has killed at least 237 people, including 21 U.S. service members, in the past nine days ahead of the referendum.

In Baghdad, Iraqi security forces backed by U.S. helicopters fought about 40 gunmen house-to-house Tuesday in south Baghdad. More than three dozen insurgents were killed, wounded or detained, the U.S. military said. Three Iraqi soldiers were injured.

Also Tuesday, a suicide car bomb exploded at the main entrance to the heavily fortified Green Zone, a district of Iraqi government buildings and the U.S. and British embassies. The blast killed at least two policemen.