Baghdad, Iraq ? Gunmen rounded up a Sunni family that had received death threats for joining U.S.-organized talks with local Shiites, hauling away the men and boys and killing all six Saturday as suspected insurgents expanded a campaign of fear against opponents.

The al-Qaida-affiliated group Islamic State of Iraq posted an online video of the execution of 18 Iraqi security troops, shot in the back of their heads while kneeling in a field.

The three-minute video, posted on a Web site previously used by the Islamists, said the 18 kidnapped government security forces were slain in retaliation for the alleged rape of a Sunni woman by members of the Shiite-dominated police. The video’s authenticity could not be immediately verified.

The group also claimed responsibility for the deaths of 14 policemen whose bodies were found Friday in the northeast province of Diyala.

Some of the victims were decapitated, according to an AP photographer. Their remains were cleansed in Muslim tradition before burial in the Shiite holy city of Najaf, 100 miles south of Baghdad.

U.S. forces, meanwhile, reported airstrikes and raids on what it called Sunni militant bases linked to al-Qaida in Iraq.

A recent wave of Sunni reprisals appears linked to increasingly high-profile attempts to stir popular momentum against Sunni extremists trying to drive out the Shiite-led government and its American backers.

Among those targeted include a range of Sunnis raising their voices against violence: imams, clan-based vigilantes and activists trying to bridge deep rifts with majority Shiites.

“We are seeing more people beginning to challenge the insurgents,” said Marine Brig. Gen. John Allen, who oversees units in the militant heartland west of Baghdad.

The two families gunned down at sunrise Saturday had received death threats for weeks after attending gatherings of Sunni and Shiite leaders, police said.

The first meeting, organized by U.S. military officials on Feb. 13, brought together leaders of prominent clans from both sides, said military spokesman Maj. Webster M. Wright III.

The clan chiefs held another round on their own about a week later and appointed a joint council “to discuss the terms of reconciliation” around Youssifiyah, a Sunni-dominated area about 12 miles south of Baghdad, Wright said.

At dawn, gunmen stormed the home of two families belonging to the influential Sunni Mashhada tribe, said police 1st Lt. Haider Satar. Two fathers and their four sons were separated from their wives and sisters. They were executed at point-blank range.

In the morgue in nearby Mahmoudiya, AP Television News footage showed at least two victims had their hands bound behind their backs.