"Islamic State of Iraq and the Levant" (ISIL) 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 said Tuesday (December 27th).

The killings took place in the villages of al-Hud and al-Lazzagah, 50 kilometres south of Mosul on October 17th, the day government forces launched the massive operation to oust the group from the city, AFP reported.

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

In total, the ISIL 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.

ISIL 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 October 17th, about 30 villagers attacked the jihadists, killing 19 of them, said the watchdog.

ISIL 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.