About 1,000 U.S. Marines (search) and Iraqi soldiers have launched a major operation in western Iraq to disrupt insurgents and foreign fighters in the Euphrates River valley, the U.S. military said Friday.

The operation, dubbed Quick Strike, began Wednesday with Iraqi soldiers and Marines positioning their units, said a military statement. They are focused on an area centered around the cities of Haditha (search), Haqlaniyah and Parwana, about 140 miles northwest of Baghdad.

On Wednesday, 14 Marines and their civilian translator were killed when their vehicle was hit by a roadside bomb near Haditha as they traveled inside a lightly armored vehicle.

On Friday, U.S. and Iraqi troops, including Special Operations (search) forces, moved into the city of Haqlaniyah, the Marine statement said. U.S. jets attacked insurgents hiding in buildings outside the town.

Residents in the area said U.S. and Iraqi forces had cordoned off Haqlaniyah and were conducting house-to-house searches. U.S. warplanes circled overhead and a number of heavy explosions were heard. Witnesses said 500-pound bombs were being dropped in the area.

The U.S. military has defended its operations in western Iraq, insisting it is reducing insurgent attacks, despite the deaths of the 14 Marines. The extremist Ansar al-Sunnah Army claimed responsibility in a Web posting and said its fighters used two bombs to destroy the vehicle.

In Najaf, Prime Minister Ibrahim al-Jaafari said Iraq's most influential Shiite Muslim cleric would like the new constitution to enshrine Islam as the main source of legislation.

That is opposed by Kurds and some Iraqi women activists, and is one of issues that political leaders are trying to resolve so a proposed constitution can be presented to parliament by the Aug. 15 deadline.

The views of Grand Ayatollah Ali al-Sistani hold considerable sway among Shiite Muslims, who make up 60 percent of Iraq's 27 million people.

After meeting with the cleric, al-Jaafari said: "Ayatollah al-Sistani does not want to impose dictation on drafting the constitution, but according to my knowledge he hopes that Islam become the main source of legislation."

Four more U.S. service members were killed in action Wednesday, the military said — three in Baghdad and one in Ramadi.

A U.S. military spokesman, Brig. Gen. Donald Alston, said American military operations in Anbar province, which includes the area where the Marines died, have succeeded in disrupting insurgent activities.

"We still have deaths. We still have suicide car bombs," he said. "But the numbers we see indicate [the insurgents] can't generate the same tempo, and I think that's because we've had some degree of effect in interdicting these forces."

Alston cited figures showing there were 13 car bombs in Iraq last week — the lowest weekly number since April. "There's a clear indication to me that the tempo has decreased."

U.S. troops have stepped up operations in recent months in Anbar, the center of the Sunni Arab-dominated insurgency and a major avenue for foreign fighters infiltrating the country from Syria.

Alston warned that militants will likely rally their forces in a concerted effort to derail the country's political progress, including a referendum on the constitution in October and an election in December.

The president's office said a key meeting scheduled for Friday by political leaders to hammer out differences in the draft constitution has been postponed until Sunday. The statement issued Friday did not say why the meeting was delayed.

The gathering was called by constitutional committee chairman Humam Hammoudi, who promised the National Assembly that the draft charter would be ready by the Aug. 15 deadline, provided the country's political leaders reach compromises on key issues including federalism, the role of Islam, and distribution of national wealth.

U.S. leaders, who pushed hard for the committee not to seek an extension on completing the charter, considers the constitutional process vital to maintain political momentum, undermine the insurgency and pave the way for the Americans and their coalition partners to draw down troops next year.

U.S. commanders have warned that although the number of vehicle and roadside bombings are decreasing, they are increasing in potency and sophistication. Bombs on the roads or planted in vehicles account for 70 percent to 80 percent of the U.S. deaths in Iraq, command spokesman Lt. Col. Steven Boylan said.

A roadside bomb late Wednesday killed three U.S. soldiers in Baghdad, the U.S. command said. A Marine was killed Wednesday by small arms fire in Ramadi, the capital of Anbar province 70 miles west of Baghdad, the command added.

At least 1,826 members of the U.S. military have died since the Iraq war started in March 2003, according to an Associated Press count.

Al-Jaafari on Thursday announced a new 12-point security plan. He gave few details but said it included steps to improve intelligence, protect infrastructure and prevent foreign fighters from entering the country.

"We will not hesitate in saying this: We are in a state of war. It is one of the most dangerous types of war because it is not a conventional or a war of borders," he said.