The United Nations’ commissioner for human rights said Monday the Islamic State of Iraq and Syria “almost certainly” committed war crimes by executing hundreds of civilians and disarmed soldiers in the last five days.

"Although the numbers cannot be verified yet, this apparently systematic series of cold-blooded executions, mostly conducted in various locations in the Tikrit area, almost certainly amounts to war crimes,” Navi Pillay said, according to Reuters.

ISIS released graphic photos showing what it said were the executions of hundreds of captured Iraqi soldiers.

Pillay’s comments came a day after U.N. Secretary-General Ban Ki-moon condemned the violence by ISIS in Iraq as the Sunni militant group upped attacks on Iraqi security forces last week, seizing the country’s second-largest city, Mosul.

“The secretary-general reiterates his condemnation of the recent upsurge of violence in Iraq at the hands of terrorist groups including” ISIS, a spokesman for Ban said in a statement Sunday. “Reports of mass summary executions by [ISIS] are deeply disturbing and underscore the urgency of bringing the perpetrators of such crimes to justice.”