U.S. and Iraqi forces struck back at insurgents Friday, storming a cluster of northwestern towns in a region where 22 Marines have been killed this week.

The assault near the city of Haditha, in violence-plagued Anbar province, came as politicians seeking to complete a draft of a new Iraqi constitution by Aug. 15 postponed until Sunday a meeting scheduled to work through several remaining sticking points.

The delay came after Massoud Barzani, the president of the Kurdish area of northern Iraq who is pushing for greater regional autonomy, decided to remain in the Kurdish capital of Irbil to huddle with his parliament before traveling to Baghdad to negotiate.

The assault in Anbar began before 6 a.m., as 180 Iraqi soldiers and 900 members of the 2nd Marine Division, backed by M1-A1 Abrams tanks, helicopters and jets bearing 500-pound bombs, stormed into Haditha, Haqlaniyah and other small towns along the Euphrates River where military officials say insurgents are mustering to launch attacks. Iraqi special operations forces helped direct U.S. airstrikes Friday morning, according to a military news release.

The Marines have conducted at least half a dozen similar offensives in the province since early May. But the insurgency has remained strong in the Sunni Arab-dominated region, where opposition to the U.S. military presence in Iraq runs deep.

"When we did operations out west, the insurgents moved to the Haditha-Haqlaniyah area," said Col. Bob Chase, the operations chief for the 2nd Marine Division. "It has good urban terrain for them to melt into. And we are well aware there are still plenty of ammunition supplies from the Saddam Hussein era for them to make use of."

The Marines were positioning for the initial stages of what they have dubbed Operation Quick Strike on Wednesday when 14 were killed as an amphibious assault vehicle struck a roadside bomb near Haditha, a city of about 70,000.

Chase said the roadside bombing, the deadliest of the war, and another attack Monday in which six Marines were killed by small-arms fire, were still under investigation. There is "no evidence whatsoever" that insurgents conducting the attacks were tipped off by Iraqi forces, he said.

The explosion, which was so powerful that it flipped the 30-ton vehicle, was caused by a stack of three anti-tank mines, the American Forces Press Service, a Defense Department organ, reported Friday, citing an unnamed senior defense official.

During Friday's operation, there was sporadic resistance from insurgents wielding small arms and rocket-propelled grenade launchers. No casualties were reported, but a potentially dangerous situation was averted when Marines discovered a pair of buildings linked by trip wires and with 155mm artillery rounds scattered throughout. The buildings were leveled.

In a statement posted in a mosque in Haditha, the insurgent group known as al Qaeda in Iraq said it would avoid fighting Marines inside the city because it feared civilians would be hurt.

Elsewhere Friday, efforts continued to reach a compromise in the constitution-writing process.

Prime Minister Ibrahim Jafari met with Iraq's most influential Shiite Muslim cleric, Grand Ayatollah Ali Sistani, in the southern city of Najaf. Jafari was seeking support for two controversial constitutional provisions: that Islam is a main source of Iraqi law, and that the country be governed through a federal system in which some power is devolved to regions.

After their two-hour meeting, Jafari said Sistani endorsed both views. Sunni Arabs, who represent an estimated 20 percent of Iraq's population, favor limiting federalism to Kurdistan and oppose the formation of a Shiite state in the south. Kurds, who are predominantly Sunni but whose politics are largely secular, oppose granting Islam a major role in determining Iraqi law.

"The only issues remaining are those over which the leaders of the blocs will be deliberating soon," said Shiite Sheik Jalaladeen Sagheer at Baghdad's Buratha mosque. "We are surging ahead."

Sarhan reported from Najaf. Correspondent Ellen Knickmeyer and special correspondents Omar Fekeiki, Bassam Sebti, Naseer Nouri and Khalid Saffar in Baghdad contributed to this report.

Marine Lance Cpl. Jeremiah Crosby, 20, takes a break near Haditha. His hand rests on a photo of his wife.