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The UN has said there are reports of dozens of "execution-type killings" by Islamic State group militants, including the slaying of 50 former police officers, as Iraqi troops close in on Mosul.

The allegations - which remain "preliminary" - have come from a range of civilian and government sources, who cannot be named for security reasons, said United Nations rights office spokesman Rupert Colville.

The reported atrocities were perpetrated by the militants between Wednesday and Sunday, while Iraqi forces advanced towards Mosul, the last IS bastion in the country, Colville said.

In a village called Safina, about 45 kilometres (28 miles) south of Mosul, IS was blamed for executing 15 civilians before throwing their bodies in a river, possibly to strike terror among other residents.

[IS] tied six civilians to a vehicle by their hands and dragged them around [a] village. - UN's Rupert Colville

On 19 October, also in Safina, extremist fighters "reportedly tied six civilians to a vehicle by their hands and dragged them around the village, apparently, simply because they were related to a particular tribal leader fighting against [IS]", Colville said.

Iraqi security forces found another 70 bodies riddled with bullet wounds on 20 October in the nearby Tuloul Naser village. Colville said it was not immediately clear who was responsible for their deaths.

And on Saturday, IS gunmen allegedly shot dead three women and three girls during a forced march in Rufeila village south of Mosul.

The group was killed because they were struggling to keep up, likely because one of the girls who was ultimately shot dead had a physical disability, the rights office said.

The 50 police officers who had been held hostage by IS were reportedly executed in a building outside Mosul on Sunday, Colville told reporters in Geneva.

"We very much fear that these will not be the last such reports we receive of such barbaric acts by ISIL," he said.

He added that all the allegations "need a bit more (investigative) work" before the UN can conclusively say they took place.

The rights office also restated its fears that IS will use civilians in Mosul as human shields as Iraqi forces fight to retake the city in an operation backed by a US-led coalition.