Islamic State has summarily killed 21 civilians in Mosul it accused of collaborating with U.S.-backed security forces, which said their offensive to recapture the city from the jihadists had made further advances.

The killings, reported on Tuesday by a medical source, indicate that the ultra-hardline group has maintained its ability to police Mosul, more than four weeks after the start of the offensive on the northern Iraqi city.

The campaign that began on Oct. 17 is the biggest military operation in Iraq in more than a decade of turmoil unleashed by the 2003 U.S.-led invasion that toppled Saddam Hussein.

Counter terrorism forces breached Islamic State defenses in the east of the city two weeks ago but have faced resistance from the militants who have deployed suicide car bombs, snipers and waves of counter-attacks.

They have been fighting in a dozen of the roughly 50 neighborhoods on the eastern side of the city, which is divided in two by the Tigris river running through the center.

Interior Ministry spokesman Saad Maan said on Tuesday that Iraqi forces in the east of the city, which also include part of a tank division, had dislodged Islamic State fighters from a third of the eastern side of the city.

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