The Al Qaeda-affiliated group Islamic State of Iraq posted an online video Saturday 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 tape firsts depicts the 18 men, some in Iraqi military uniforms, blindfolded, hands tied behind their backs and lined up in three rows in front of a screen. The men in the front row are kneeling down. Armed masked men were seen pointing machine-guns at the captives.

Male voices chant repeatedly in Arabic during the video: "At your service sister" — a likely reference to the revenge for the allegedly raped Sunni woman.

Another male voice is heard reading the Islamic State of Iraq's statement posted on Friday on the same Web site, saying the group's court has ordered the 18 security troops executed because Iraqi Prime minister Nouri al-Maliki's government failed to meet their demands and hand over the officers who allegedly assaulted the Sunni woman, and to release all female detainees from Iraqi prisons.

Then the video moves to the scene of execution with the men lined up kneeling on the ground in a field by a citrus grove, with date palm trees in the background. They are still blindfolded, hands tied in the back.

Two masked militants, with checkered kaffiya scarves on their heads, fire from close range into the back of the heads of the men from handguns, as a third militant carries a banner ahead of them. The victims fall head forward to the ground. The spray of bullets is accompanied by chants "Allahu Akbar," or God is the Great.

Earlier Saturday, the same group posted a text statement on the same Web site, saying that the 18 security troops were not the same as 14 Iraqi policemen whose bodies were found northeast of Baghdad on Friday.

The group claimed it had seized on Thursday and killed the 14 policemen in a different operation from the capture of the 18 troops. The policemen were also slain in retaliation for the alleged rape of the same Sunni woman, it said.

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.

Elsewhere, 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.

U.S. forces, meanwhile, reported airstrikes and raids on what it called Sunni militant bases linked to Al Qaeda 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.

For the second consecutive day, just one major explosion rocked the capital. The latest — a roadside bomb — killed three U.S. soldiers on patrol in central Baghdad, the military said.

In western Baghdad, meanwhile, a top adviser to Iraq's Defense Ministry was kidnapped in western Baghdad, said an aide to Adnan al-Dulaimi, a leader of the largest Sunni bloc in parliament.

Lt. Gen. Thamir Sultan hails from Saddam Hussein's tribe and had been mentioned as a possible defense minister when the current government was organized last year. The aide spoke on condition of anonymity because he was not authorized to release the information.

In U.S. raids north of Baghdad, nine suspected insurgents were captured, including two believed to be responsible for recruiting and helping foreign militants in Baghdad, the U.S. military said. The suspects were also accused of harboring Al Qaeda in Iraq leaders, it said, but gave no further details.

U.S. warplanes also struck a suspected car bomb factory in Arab Jabour, south of Baghdad, the military said. Seven suspected insurgents were killed when two precision-guided bombs destroyed the structure where they were hiding, the military said.