The victims included the imam of the Grand Mosque in Mosul when the imam refused to pledge allegiance to ISIS. Twelve other imams were also executed in front of Al Israa mosque, Pillay said.
ISIS has claimed to have executed 1,700 conscripts after they took over the city of Tikrit.
Iraqi forces have been struck back with helicopters launching rockets at targets near the main road leading to Baghdad. The city was rocked by a string of explosions, with as many as 15 dead, Iraqi police and hospital officials said.
Top Republicans, such as South Carolina Sen. Lindsey Graham, say action is needed.
"If Baghdad falls, if the central government falls, a disaster awaits us of monumental proportions," Graham said Sunday on "State of the Union."
The Associated Press contributed to this report.I