So-called Islamic State group fighters "summarily executed" 13 civilians after villagers rose up against them at the start of the Iraqi army's offensive to retake Mosul, Human Rights Watch has said.

The killings took place in the villages of Al-Hud and Al-Lazzagah, 50km south of Mosul on 17 October, the day government forces launched the massive operation to oust the jihadists from the city.

"ISIS responded to the village uprising by unlawfully executing people captured in the uprising and civilians who weren't involved," Lama Fakih, deputy Middle East director at HRW, said in a statement.

"Security forces who capture ISIS fighters should properly investigate their participation in alleged war crimes like these," she said, using an alternate acronym for the jihadist group.

In total, the IS fighters "summarily executed at least 13 people including two boys," HRW said.

The report included the picture of one of the slain boys, a 13-year-old who had not been involved in the uprising, it said, citing his father Muhammad.

IS had captured Al-Hud and Al-Lazzagah in June 2014, with villagers saying they lived in constant fear of punishment, including death, for activities like smoking and using mobile phones, said HRW.

As Iraqi forces closed in on the morning of 17 October, about 30 villagers attacked the jihadists, killing 19 of them, said the New York-based watchdog.

IS fighters began the execution-style killings in the afternoon, leaving bodies lying in the streets.

Iraqi forces entered Al-Lazzagah that evening and Al-Hud the next morning.

Human Rights Watch called on Iraqi security forces to "appropriately investigate incidents of alleged war crimes so that those responsible, if in government custody, can be fairly prosecuted".

After seizing control of large parts of Iraq and neighbouring Syria in mid-2014, IS declared a cross-border "caliphate", imposed its harsh interpretation of Islamic law and committed widespread atrocities.

Iraqi forces have been tightening the noose around Mosul since launching the offensive.