Diyala police spokesman Maj. Ghalib al-Karkhi said the government-allied fighters, known as Sahwa or Awakening Councils, were driving near the town of Muqdadiyah around 1:30 a.m. when their car hit a roadside bomb.
The explosion killed four of the guards immediately, al-Karkhi said. Gunmen then attacked the two survivors, killing them, he said.
Muqdadiyah is about 60 miles (90 kilometers) north of Baghdad.
The Sahwa are a government-backed Sunni militia that joined forces with U.S. troops against al-Qaida in 2006 and helped turn the tide of the war. Since then, the Sahwa fighters have become frequent targets of insurgent attacks.
The early morning ambush comes on the heels of a string of attacks a day earlier that struck at least 13 Iraqi cities. Fifty-six people were killed - including at least 31 policemen and soldiers - in a series of bombings and shootings across the country.
The attacks made August the deadliest month for Iraqi security forces in two years. They came after the U.S. declared the number of U.S. troops had fallen to fewer than 50,000, their lowest level since the war began in 2003.
At least 265 security personnel - Iraqi military, police and police recruits, and bodyguards - have been killed from June through August, compared to 180 killed in the previous five months, according to an Associated Press count.
On Wednesday, as in earlier attacks, insurgents took aim at security forces where they are supposed to be the safest, turning police stations into rubble and bringing down concrete walls erected to protect them.
Iraq's foreign minister said insurgents are attempting to sow as much chaos as possible, as lawmakers struggle to form a new government and Americans withdraw troops.
"Here you have a government paralysis, you have a political vacuum ... you have the U.S. troop withdrawal," Iraqi Foreign Minister Hoshyar Zebari said. "And, in such environment, these terrorist networks flourish."
In August, nearly 5 Iraqi security personnel on average have been killed every day so far.
These numbers are considered a minimum, based on AP reporting. The actual number is likely higher, as many killings go unreported or uncounted.
That rise in deaths coincided with the drawdown of U.S. troops. American officials said on Tuesday that the number of troops fell below 50,000 - a step toward a full withdrawal by the end of 2011.
A senior Iraqi intelligence official, who spoke on condition of anonymity because he wasn't authorized to talk to the media, raised the possibility that some of the attackers had inside help.
Associated Press Writer Lara Jakes contributed to this report.