It's becoming an increasing preoccupation for the U.S.-led coalition, said an American official, who spoke on background prior to Prime Minister Stephen Harper's trip to Baghdad, and requested anonymity because of the sensitivity of the topic.
There have been multiple reports of rampage killings of Sunnis considered collaborators in Tikrit, Amerli, and Jurf al Sakhar, all of which were freed by Iraqi forces and Shiite-backed militias with the help of coalition air power.
Washington is also apparently particularly disturbed by reports Hezbollah Brigades — a designated terrorist group — joined the fight last month to keep Ramadi, in western Iraq, out of extremist hands and about the activities of a group known as the Popular Mobilization Committee, said the official.
Human Rights Watch has accused militias associated with the committee of war crimes.
Iraqi Yazidis returning to their homes in northern Iraq also joined the mayhem, conducting atrocities of their own neighbours whom they suspected of driving them out last summer, a Canadian official acknowledged recently.
The question of whether the wave of revenge attacks will grow even more fierce haunts western military planners as they weigh how and when to retake Iraq's second-largest — mostly Sunni dominated — city of Mosul.
"It's going to be ugly for a while yet," said the Canadian official, who was not authorized to speak to the media.
Defence Minister Jason Kenney is fond of describing the Islamic State in Iraq and the Levant as a "genocidal death cult," but often ignored in the overheated rhetoric is the flip side of the sectarian coin in this brutal, little corner of the globe.
The war against ISIL is packaged and portrayed by the Harper government in purely domestic — some have argued politically self-interested — terms.
"Make no mistake: by fighting this enemy here you are protecting Canadians at home," Stephen Harper told CF-18 fighter pilots and maintainers here Sunday. "Because this evil knows no borders and left uncontained, it will spread like a plague."
But amid references to ISIL's "orgy of terrorist violence around the world" and "acts of brutality" and "savagery," there was a startling moment of reflection, a partial nod to the mind-bending complexity of a region steeped in ancient, intractable blood feuds.
"Some will say we don't know how effective our actions will be, or if this is ideal strategy. And how could we?" Harper said. "But what we do know is this: In the face of this menace, the worst thing we could possibly do is nothing."
Those words have the potential to boomerang when you look at it through the growing unease over retribution killings.
It's unclear whether Harper counselled restraint in his weekend meeting with Iraqi Prime Minister Haider al-Abadi, whose Shiite-dominated government has relied on the Iranian-trained militias that are carrying out most of the rampages. It was, after all, the brutal — sometimes murderous — policies of al-Abadi's predecessor that led to the rise of the Islamic State in the first place.
Harper's government did drop $26.5 million on stabilization and development initiatives, with about $8 million of that backdated to the last budget year. A substantial portion is for bricks and mortar and clearing away mines.
Only $1.2 million was set aside to support human rights investigations into atrocities, but the money is being split between Iraq and Syria.
Rosemary McCarney, president of Plan International Canada, which works with displaced families, says the humanitarian crisis in the region is already enormous with 2.7 million people driven from their homes.
"The scale of the humanitarian crisis is extraordinary and so the financial commitments are welcome, but we have to do more," said McCarney, who was travelling with Harper in Iraq. "Canada is uniquely placed to play a leadership role."
Her comments were echoed by Rahul Singh, of Global Medic, which is distributing aid to families who have fled the fighting.
"This a terrible situation with tons of needs," he said.
Singh says there are many people existing outside of the formal refugee camp system. They are living in incomplete, or ruined buildings — or taking refuge with religious organizations, including churches.