UN rights office spokeswoman Ravina Shamdasani told reporters in Geneva that the jihadists are forcing civilians living in districts around Mosul into the city, hoping to use them as human shields in an upcoming battle.
Those targeted in mass killings have especially included civilians who refuse relocation orders and people who previously worked for the Iraqi Security Forces (ISF), Shamdasani said.
The IS jihadists slaughtered 24 ex-ISF officers on Tuesday and another 190 on Wednesday, according to rights office reports.
The latter group were shot dead at the Al-Ghazlani military base inside Mosul, Shamdasani said.
Also on Wednesday, "42 civilians were reportedly shot in the head at the al-Izza military base" outside Mosul, a UN rights office statement said, apparently for refusing to follow the IS group's instructions.
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The rights office earlier in the week had listed a series of purported IS atrocities around Mosul but described the allegations as "preliminary" and needing more investigative work.
Shamdasani said Friday that the fresh allegations had been "corroborated", but may not reflect the full toll as there were likely other atrocities that have gone unreported.
Meanwhile, the rights office said the extremists had kidnapped nearly 8,000 families around Mosul, the majority from the Al-Shura sub-district.
"Credible reports suggest that ISIL has been forcing tens of thousands of people from their homes in sub-districts around Mosul," said Shamdasani, using another acronym for IS.
She said the jihadists were using a "depraved, cowardly strategy" in the face of a US-backed Iraqi government offensive to retake Mosul, the IS group's last bastion in Iraq.
The offensive, launched on October 17, has seen tens of thousands of Iraqi soldiers advancing on the city from the south, east and north.
Last week, the rights office reported dozens of execution-style killings in villages near Mosul, including the shooting of a physically disabled girl who failed to keep up on a forced march.