Also on Saturday, three US soldiers were killed by a roadside bomb that exploded while they were on patrol in central Baghdad, the military said.
A adviser to Iraq's defence ministry was kidnapped in western Baghdad, an aide to Adnan al-Dulaimi, the head of the largest Sunni bloc in parliament, said.
Lieutenant General Thamir Sultan hails from Saddam Hussein's tribe and had been mentioned as a possible defense minister when the current government was organised last year. The aide spoke on condition of anonymity because he was not authorised to release the information.
Earlier, the US military said it killed "key terrorists" who were using anti-aircraft artillery to fire at American military helicopters near Taji, north of Baghdad.
The Iraqi men gunned down Saturday - all relatives from the Mashhada tribe - were killed after gunmen stormed the family's house at dawn, Haider Satar, an Iraqi policeman, said.
The victims in Youssifiyah were separated from women and children and shot dead in their home, Satar said.
Captain Ahmed al-Obeidi, from the Iraqi army, whose unit is stationed near the site of the attack, also confirmed the incident.
The bodies were transferred to a hospital morgue in the neighbouring town of Mahmoudiyah, where Associated Press Television footage showed them wrapped in blankets and lined up on the metal floor of a refrigeration chamber.
The latest figures for military deaths in Iraq since the US-led invasion in March 2003:
US-led coalition forces:
• United States: 3,172
• Britain: 133
• Other nations: 124
• Military: Between 4,900 and 6,375
• Civilians: Between 57,660 and 63,428
At least two had their hands tied and bound behind their backs.
The motive of the brutal attack could not be independently verified.
But police, citing information from surviving relatives, said the victims had received threats from Sunni gunmen after participating in a reconciliation conference with Shias last month.
The conference was held in Mahmoudiyah in late February.
The US military said it captured nine suspected fighters in a raid on Saturday morning north of Taji, a town on Baghdad's northern outskirts.
Two of the men were believed to be responsible for recruiting and helping foreign militants join the insurgency in Baghdad, the military said in a statement.
The suspects were also accused of harbouring Iraq leaders and members al-Qaeda, it said.
A day earlier, US aircraft bombed an area west of Taji, targeting fighters who were armed with anti-aircraft artillery, the military said in another statement on Saturday.
"Key terrorists were killed during the air strike," the statement said.
Lieutenant Colonel Christopher Garver, a US military spokesman, said that insurgents near Taji had been firing at US helicopters with heavy machine guns mounted on the back of truck.
"It's mobile and it can inflict damage to our helicopters," Garver said.
"It's mobile and it can inflict damage to our helicopters... Anything that can threaten our helicopters, we're going to try to get it off the battlefield"
Lieutenant Colonel Christopher Garver, a US military spokesman
"Anything that can threaten our helicopters, we're going to try to get it off the battlefield," he said.
Iraq's defence ministry said on Saturday that Iraqi troops killed three suspected gunmen in Khan Bani Saad, a mixed town northeast of Baghdad.
Two men were arrested in the raid, the ministry said in a statement. Seven others were captured in Balad Ruz, 70km northeast of Baghdad, it said.
Meanwhile, Zalmay Khalilzad, the US ambassador to Iraq, wrote in a commentary published on Saturday that "resolving concerns about control of oil is central to overcoming internal divisions in Iraq."
Iraq's new oil law would "serve as a vehicle to unify" the country, Khalilzad wrote in The Washington Post.
A roadside bomb exploded on a minibus on Friday night in Latifiyah, about 30km south of Baghdad, killing two women and an 11-year-old child, Satar said.
Gunmen killed a civilian and wounded two others in separate attacks on Saturday near Hillah, about 95km south of Baghdad, Muthana Khalid, a police captain said.
Lieutenant Colonel Fadhil Rahim, an Iraqi special forces agent, was shot dead on Saturday in an ambush on his convoy in Adil, a predominantly Sunni neighbourhood in western Baghdad, police and hospital officials said.
Three gunmen killed a police officer after breaking into his house on Saturday in eastern Mosul, said an Iraqi policeman.
A roadside bomb exploded near a police patrol on Saturday morning in southeast Baghdad, wounding three policemen and one civilian, police said.