BAGHDAD, Iraq – The militants showed no mercy. They blindfolded 18 men, shot them in the head and decapitated three others in what has become a trademark of Iraq’s often savage insurgency.

Five U.S. Marines were killed in the same arid western region in a roadside bombing while conducting combat operations near a volatile Sunni town.

The latest violence near the Syrian border comes despite a major American and Iraqi military effort to drive tenacious insurgents out of Anbar province, one of Iraq’s most dangerous territories.

Also Friday, in Baghdad a car bomb killed four men and wounded nine as they sat outside a takeout restaurant.

The Iraqi victims whose bodies were found in the desert near the dusty, lawless frontier town of Qaim may have been from a group of Iraqi soldiers who went missing Wednesday after leaving their base en route for Baghdad for a vacation. Not one was in uniform and most were in casual clothes – including a track suit.

The killings were a clear sign of the profound difficulties faced by U.S. and Iraqi forces in Anbar province and their inability to seal the porous desert border with Syria despite major efforts to boost their military presence in the area.

Marines carried out two major operations in the area last month, killing 125 insurgents in the first campaign, Operation Matador, and 14 in the second, Operation New Market. Eleven Marines were killed in the actions, designed to scatter and eradicate insurgents using the road from Damascus to Baghdad.

The 21 Iraqi bodies were found near a highway that meanders along the Euphrates River and into Syria. The bodies were in three locations, haphazardly dumped by the roadside in a gravel pit and in sand flats. Three were beheaded and at least one had been mauled by animals.

Al Qaeda in Iraq claimed in an Internet posting that it abducted 36 Iraqi soldiers in western Iraq on Wednesday, and threatened to kill them unless al-Jaafari’s government released “Muslim women” from prison.

U.S. military intelligence officials believe Qaim sits at the crossroads of a major route used by groups such as Abu Musab al-Zarqawi’s al Qaeda in Iraq to smuggle foreign fighters into the country.

“It’s like the Mexican-American border there. There are attempts being made to seal it,” a senior U.S. military intelligence official said on condition he remain unnamed for security reasons.

Insurgents are so rooted in the region that after a May 29 gun battle in a village between Qaim and Haqlaniyah, U.S. forces were surprised to find the body of the kidnapped governor of Anbar province chained to a propane tank and killed by falling rubble.

As part of its effort to increase its presence, the Iraqi army boosted the number of soldiers at the frontier post of Akashat, near Qaim, from about 100 before Operation Matador to nearly 750 now. Akashat is where the missing soldiers were based.

The Marines were killed Thursday near the volatile Sunni town of Haqlaniyah, 90 miles northwest of Baghdad, the military said. Their deaths brought to at least 1,689 the number of U.S. military members killed in Iraq since the war began in March 2003, according to an Associated Press count.

Also Friday, gunmen killed the dean of the police academy in the southern city of Basra and an Iraqi soldier was killed when a roadside bomb exploded in the central city of Mashru.

The bloodshed came as politicians seeking a negotiated solution to the insurgency once again wrangled over a promise to give Sunni Arabs a bigger say in charting Iraq’s future.

Iraqi politicians were divided over President Jalal Talabani’s promise to give Sunni Arabs more seats on a 55-member committee drafting Iraq’s first postwar constitution.

The charter must be ready to present to the 275-seat National Assembly by mid-August and will go before Iraq’s voters in a referendum two months later. It requires the support of Sunni Arabs – thought to make up 20 percent of the population.

Talabani’s promise to raise the Sunni Arab representatives from a proposed 15 to 25 – increasing the committee’s size to 80 – averted a crisis after Sunni Arabs threatened a boycott.

They renewed that threat Friday if the Shiites and Kurds backed down from the promise.

“We drew a red line and said, ‘Do not choose less than 25.’ We didn’t discuss this issue with Mr. Talabani and we do consider Talabani’s announcement to be official approval,” said Yousif al-Aadhami, an official of the Sunni Endowment, a charitable institution.

“These seats are not given to us as charity,” he added. “This is what we deserve.”