Police said the bodies of the men, all apparently civilians, were found in Baghdad's violent Ghazaliya district on Tuesday. They had marks on their wrists, indicating they had been restrained.
Three other bodies were found dumped near a road in Rustamiya on the southeastern outskirts of the capital. They also had gunshot wounds to the head. Police said their identities were not known.
Since the US-led invasion that ousted Saddam Hussein in March 2003, hundreds of bodies have been found dumped in fields, at roadsides and in rivers, often bound and shot in the head.
Anti-US Iraqi fighters regularly capture and kill Iraqi soldiers, policemen and civilians suspected of working with the US-led forces in Iraq.
Iraqi Sunni Arabs, who dominate the revolt against the US-led forces in Iraq and the US-backed Shia-led government, accuse the Iraqi security forces of kidnapping their sect members and brutally torturing them before executing them. The Shia-led security forces deny the charges.
In other developments, four Iraqi soldiers were killed and another wounded after they clashed with armed Iraqi fighters in Tarmiya, a small town 30km north of Baghdad, an army official said.
It was not immediately clear if there were any casualties among the fighters.
A British soldier was killed by an explosion in the southern Iraqi port town of Umm Qasr. Britain's Ministry of Defence said three other soldiers were wounded, one seriously, in the blast.