Human Rights Watch details examples of shootings, bombings and beheadings which have claimed thousands of lives in the most extensive compilation of atrocities by rebel groups.
The dossier stresses that the killings should be put in the context of the tens of thousands who died in the US-led invasion. It also notes the anger caused by the torture of prisoners at Abu Ghraib.
But it says "none of the arguments justify attacks which are in clear violation of international law. Regardless of the violations committed by US and Iraqi [ government] forces, almost daily attacks on civilians have had a devastating impact on the people of Iraq.
"Insurgent groups have also tortured and summarily executed civilians and captured combatants in their custody, sometimes by beheading. And they have carried out attacks against legitimate military targets, such as army convoys, in such a manner that the foreseeable loss of civilian life was greatly disproportionate to the military gain."
Sarah Leah Whitson, the director of Human Rights Watch's Middle East and North Africa Division, said "Ultimately, whether the insurgency in Iraq succeeds or not will depend not on the coalition forces but on whether the insurgents lose the support of the people. That appears to be happening, many are repulsed by the behaviour of the insurgent groups even if they support the withdrawal of American troops."
The report, A Face and a Name: Civilian Victims of Insurgency Groups in Iraq, also does not address the growing number of abuses in the British-controlled south, where the dominant Shias have been carrying out a campaign of intimidation against Sunnis and Christians with British forces accused of turning a blind eye in order to avoid confrontations.