Amnesty International said Thursday it was investigating reports of serious human rights violations committed by Iraqi government and allied forces in the operation to retake the city of Tikrit.
"We are very concerned by reports of widespread human rights abuses committed in the course of the military operation in the area around Tikrit," the rights watchdog's Donatella Rovera told AFP.
Security forces backed by paramilitary groups and supported by US-led air strikes retook Tikrit from Islamic State over the past few days.
Outlying areas in Salaheddin province, which had also been under ISIS control since 2014, were retaken gradually over the course of the past month.
The operation, Baghdad's largest yet against the jihadists, was seen as a test of the Shiite-dominated forces' ability to retake a Sunni area while reining in reprisals against the local population.
"We are investigating reports that scores of residents have been seized early last month and not heard of since, and that residents' homes and businesses have been blown up or burned down after having been looted by militias," said Rovera, senior crisis response adviser at Amnesty.
"There have also been reports of summary executions of men who may or may not have been involved in combat but who were killed after having been captured" when not in combat, she said.
Rovera said the latest such report was an incident Wednesday inside Tikrit.
The government and its coalition partners, the United Nations and rights groups have repeatedly stressed that any military victory against ISIS that comes with sectarian-driven abuses would only sow the seeds of future violence.
Pro-government militiamen could be seen looting shops in central Tikrit Wednesday as Iraqi forces sought to consolidate control over the city.