BAGHDAD, Iraq - Militants killed five U.S. Marines and authorities found 21 bodies Friday near the Syrian border, where American and Iraqi troops bore down in two major operations aimed at crushing a tenacious insurgency.

The victims, thought to be missing Iraqi soldiers, were shot repeatedly in the head and found blindfolded, their hands tied behind their backs. Three were beheaded.

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

Also Friday, a car bomb killed four men and wounded nine as they sat outside a restaurant in Baghdad waiting to pick up falafel sandwiches, a popular Arab staple made with fried chickpeas.

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.

The Marines were killed Thursday in a roadside bombing while conducting combat operations 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.

At least 37 U.S. military members have been killed by roadside bombs since the makeup of Iraq's new government was announced April 28, according to the AP count.

The 21 Iraqi bodies were found near Qaim, 80 miles west of Haqlaniyah, along 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.

U.S. military intelligence officials believe the Qaim area sits at the crossroads of a major route used by groups such as Abu Musab al-Zarqawi's al-Qaida 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.

The bodies were thought to be those of off-duty Iraqi soldiers who left their base near Qaim two days earlier in civilian clothes aboard two minivans, headed to Baghdad for a vacation.

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.

Insurgents are so rooted in the region that after a May 29 gunbattle 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.

Intelligence officials say foreigners undergo rigorous religious indoctrination before being brought into Iraq and sent on suicide missions.

At least 91 car bombings, most carried out by suicide attackers, have been reported since Prime Minister Ibrahim al-Jaafari's Shiite-led government was announced, according to an AP count. They are responsible for at least 291 of the nearly 900 Iraqis killed since then. The insurgent death toll is difficult to determine, but is thought to number more than 270.

Al-Qaida 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.

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.

In Baghdad, Iraqi politicians were divided over 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 the Sunni Endowment - a charitable institution.

The military announced it was investigating as a criminal case the deaths in Tikrit of two U.S. Army officers killed Tuesday evening.

The deaths were initially thought to be caused by incoming artillery or mortar rounds fired from a location some distance away.

- Associated Press reporter Mohammed Barakat contributed to this story from Qaim, Iraq.