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 recent 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.

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.

The 21 Iraqi bodies were found near Qaim, 80 miles west of Haqlaniyah, along a highway that meanders into Syria.

U.S. military intelligence officials think 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.

An Iraqi Army soldier rides on patrol past a mosque with graffiti that reads Down

“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 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.

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 chose 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 with the Sunni Endowment – a charitable institution.