The exact motive for the attack was unclear, but it could be a case of insurgents killing locals allied to the central government or an internal struggle among the region's fractious tribes ahead January's elections.
The victims included a member of the country's main Sunni political party and several of his relatives, said party official Mohammed Iqbal, suggesting a political motive to the attack.
While members of the Iraqi military have been accused in the past of taking part in extra-judicial killings, such uniforms are also widely available on the open market and have been used by insurgents in the past to conceal their identities.
Violence has dropped dramatically in the predominantly Sunni regions of western Iraq after local tribes, many of whom had been involved in the anti-U.S. insurgency, banded together in so-called Awakening Councils and turned on their former allies, the militant group al Qaeda in Iraq.
The mayor of Abu Ghraib, Shakir al-Zubaie, told The Associated Press that none of the people killed in the nearby village of al-Saadan were members of the local Awakening Council, but said some had fought against al Qaeda for a short time in early 2008.
The Iraqi Islamic Party condemned the killings, describing the deaths as a revenge attack against people who had helped stabilize the area.
"The IIP condemns this ugly crime and it is a worrisome indication that the situation might be deteriorating and it represents a revenge against the people who had helped stabilize the area," the statement read. "This heinous crime reminds us of the crimes committed during the past years of insecurity. The people behind this crime are aiming at seeding insecurity and turmoil in the area."
All 13 had been shot in the head, possibly execution-style, as well as elsewhere in their bodies, said Waleed al-Zubaie of the police.
All the local officials confirmed the gunmen were wearing military uniforms.
A Defense Ministry spokesman was not immediately available for comment.
Maj. Gen. Qassim al-Moussawi, head of Baghdad's Operations Command, said in a statement that officials are looking into the 13 deaths and suspect it may have been a tribal dispute.
Speaking to the AP later, he said officials are looking for 10 suspects believed to be involved in the incident, who live in the area.
At least seven bullet-riddled bodies were admitted to the Abu Ghraib hospital, said Mohammed Sadoun, an official there.
A witness who did not want to be identified for fear of retribution, said he went to the cemetery where the bodies were dumped and saw at least 12 bodies, ranging in age from 25 to 50 years old.
Meanwhile, in the northern city of Kirkuk a parked car bomb exploded in a market, killing five people and wounding seven others, said Kirkuk Police spokesman Col. Sarhat Qadir.
The violence comes amid increasing confusion over national polls planned for January after that paves the way for the vote unless more seats are guaranteed for Iraqis living abroad, most of whom are Sunni Arabs like al-Hashemi.
He gave lawmakers until midday Tuesday to make the necessary changes to the law.
The head of Iraq's electoral commission, Faraj al-Haidari, met with al-Hashemi Monday to discuss the vice president's reservations with the bill.
Al-Haidari told The Associated Press that he warned the vice president that if he vetoes the legislation, elections will not be held as planned.
According to the Iraqi constitution, the vote must take place before the end of January 2010, and any delay to the elections could plunge Iraq into a constitutional crisis. The election date has not yet been set.
Al-Haidari said the electoral commission and parliamentary leaders would meet to try to hash out possible solutions to al-Hashemi's demands on Tuesday.
By Associated Press Writer Hamid Ahmed; APwriter Sameer N. Yacoub contributed to this report