HAQLANIYAH, Iraq - Some 2,500 U.S. troops along with Iraqi forces launched their second major offensive in western Iraq in a week Tuesday, sweeping into three towns to take them back from insurgents who had killed Marines there last month.
The U.S. military announced its first casualties of the offensives, with four troops killed by roadside bombs during the fighting and a fifth elsewhere.
The assaults in western Iraq aim to put down al-Qaida in Iraq and other Sunni-led insurgent groups that have waged a campaign of violence aimed at wrecking a crucial Oct. 15 national vote on a new constitution. The United States has hoped the new charter would bring together the country's fractious communities, but Sunnis sharply oppose it.
Sunni Arab moderates threatened Tuesday to boycott the voting after the Shiite-led parliament passed new rules over the weekend that make it effectively impossible for Sunnis to defeat the charter at the ballot box.
The new rules deepened alienation over the political process among Sunnis, who had decided to participate in the referendum but to vote no - but who now said the Shiites were using their dominance to stack the deck against them. A boycott would undermine the referendum's legitimacy and strike a blow to hopes that political progress would weaken Sunni support for the insurgency.
"Boycotting the referendum is a possible option ... because we believe that participating in the voting might be a useless act," said Saleh al-Mutlaq, a leading Sunni politician.
The United Nations also expressed concern about the new electoral rules, saying they don't meet international standards. U.N. officials have been meeting with Iraqi authorities and are confident that Iraq will ultimately agree to sound electoral rules, spokesman Stephane Dujarric said.
"Ultimately, this will be a sovereign decision by the Iraqis and it's up to the Iraqi National Assembly to decide on the appropriate electoral framework," Dujarric said. "That being said, it is our duty in our role in Iraq to point out when the process does not meet international standards."
In Baghdad, a suicide attacker set off a car bomb at the main entrance to the heavily fortified Green Zone, a district of Iraqi government buildings and the U.S. and British Embassies. The powerful blast killed two policemen.
The attack came on the first day of Ramadan, the holy Islamic month of fasting. Al-Qaida in Iraq called on its followers to step up attacks against U.S. and Iraqi forces and make it a "month of victory for Muslims and a month of defeat for the hypocrites and polytheists."
Previous Ramadans since the invasion and occupation of Iraq two years ago saw a spike in violence in Iraq - especially suicide attacks, in part because some Islamic extremists believe those who die in combat for a holy cause during the period are especially blessed.
In another statement, al-Qaida in Iraq urged Sunnis to boycott the referendum, saying U.S. authorities would fix the vote. "You know very well that the Americans are going to supervise collecting the voting boxes and counting the votes," it said.
The military launched its latest offensive in a cluster of cities in the Euphrates River valley about 140 miles northwest of Baghdad. Code-named "River Gate," it was the largest U.S. offensive in the troubled Anbar region of western Iraq this year, the military said. It also included hundreds of Iraqi troops, the largest such contingent of any of the offensives this year.
Airstrikes by U.S. warplanes and dozens of helicopters set off explosions that lit up Haqlaniyah, Parwana and Haditha before dawn Tuesday. Barrages of gunfire also were seen in the night sky. Large sections of Haqlaniyah's power were knocked out.
Some of the strikes took out bridges across the Euphrates in the area to prevent militants from escaping over them into the desert, said Lt. Col. Christopher Starling, the operations officer in Regimental Combat Team 2, which is leading the offensive.
Dozens of roadside bombs were encountered on the main arteries into the towns as U.S. troops moved in, Marine commanders said. Later in the day, U.S. snipers took positions on rooftops in Haqlaniyah as troops blared warnings on loudspeakers ordering residents to stay inside their homes, witnesses said.
The military launched a similar offensive on Saturday, 93 miles upriver, by the Syrian border. Operation "Iron Fist," which continued Tuesday, concentrated in the towns of Sadah, Karabilah and Rumana, aiming to uproot al-Qaida in Iraq insurgents who receive reinforcements and supplies from Syria. At least 57 militants have been killed in that operation.
The military said a Marine was killed Monday by a roadside bomb in Karabilah, the first U.S. death in Operation Iron Fist. In the hours before Operation River Gate began, a roadside bomb hit U.S. troops in Haqlaniyah on Monday, killing three, the military said.
Elsewhere, a soldier was shot Monday morning near Taqaddum, a town close to the city of Fallujah, also in Anbar but away from the two offensives, the military said.
The killings raised to at least 1,941 the number of U.S. military members who have died since the beginning of the war in 2003, according to an Associated Press count.
The towns of Haqlaniyah, Parwana and Haditha, with a combined population of 100,000, have no Iraqi police or troops based in them, leaving their streets open to roving insurgent groups.
On Aug. 1, an ambush by insurgents in Haditha killed six U.S. Marine snipers, and a large roadside bomb on the outskirts of the city on Aug. 3 killed 14 Marines and an Iraqi interpreter.
Last spring, Haditha General Hospital, the region's largest, was heavily damaged by a suicide car bomb that set fire to the building, and insurgents used staff and patients as human shields during fighting with Marines that followed.
In addition, the U.S. military has said that Iraq's most wanted terrorist, Abu Musab al-Zarqawi, the head of Al-Qaida in Iraq, once had a home in Haditha.
Earlier this year, hundreds of U.S. forces conducted individual sweeps in the three towns.
The newest operation is "step forward to eliminating insurgents and giving the country back to the Iraqi people," said Col. Stephen W. Davis, who added it would help residents in the Haditha area freely vote in the constitutional referendum.
On Monday, the United Nations announced in New York that it is distributing millions of copies of the draft constitution in this country ahead of the referendum. But residents in Baghdad and several other provinces told the AP on Tuesday that they have not received the document or seen it being handed out in their areas.
Under referendum rules, the constitution is defeated if two-thirds of voters in any 13 of Iraq's 18 provinces vote "no." Leaders of the country's Sunni Arab minority - which holds majorities in four provinces - were hoping to use that to knock down a draft charter they say will tear Iraq into Shiite, Kurdish and Sunni fragments, leaving the Sunnis the weakest.
But the Shiite-led parliament ruled Sunday that two-thirds of registered voters must vote "no" - not two-thirds of those who cast ballots, making it nearly impossible for Sunnis to reach that mark.