BAGHDAD/QAYYARA, Iraq (Reuters) - 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.

He was speaking at a joint news conference with Iraqi officials and officers from the Western coalition which is backing them with air strikes and soldiers on the ground, who are officially designated as advisers.

Australian Brigadier-General Roger Noble, asked about Maan’s comments, said: “The way I’d describe it is they’re well into the city on the eastern side”.

“We haven’t had out the measuring stick and done the percentages,” he told Reuters at the Qayyara military base south of Mosul which is the main base of the military campaign.

“Once you get into the city and the enemy being the way they are, with civilians, it’s a difficult fight, but fundamentally it’s on track,” he said.

Iraqi officials say the militants have used the city’s more than 1 million remaining residents as human shields, firing from rooftops of inhabited houses and using a network of tunnels to launch ambushes in the midst of residential areas.

SWIFTER KILLINGS

While the presence of civilians has slowed the advance, Iraqi officials say some of their operations have been assisted by information provided by residents about Islamic State military positions in the city.

Trying to stop the flow of any information out of Mosul, the militants have cracked down on communications, banning the use of mobile phones and also confiscating satellite dishes to prevent people from seeing the progress made by Iraqi forces.

Most of those killed in the last three days were accused of cooperating with the Iraqi army, said the medical source who saw the list of 21 names and was briefed on the charges they faced.

The city’s forensic medical department was notified of the killings, the source said, but this time no bodies emerged - unlike last week when 20 corpses were strung up across the city in a public warning against helping the army.

The militants who have ruled Mosul since they swept through northern Iraq two years ago have imposed a ruthless authority across the city, crushing dissent, killing opponents and forcing people to follow their strict interpretation of Islamic law.

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Residents say most victims are disposed of in mass graves outside Mosul because the militants deem their opponents apostates unworthy of religious funerals. Families only learn the fate of relatives from lists delivered to medics at the morgue.

Before the offensive started, people detained by Islamic State in Mosul were investigated for months. Now they were held for as little as two weeks before being released or killed - either shot, their throats slit, or put to death by other methods, the source said.

“HUNDREDS KILLED” ON SOUTHERN FRONT

The accelerated killings come as Iraqi counter terrorism forces are battling to expand their foothold in the city’s eastern neighborhoods.

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Iraqi military estimates put the number of Islamic State fighters in the city at 5,000 to 6,000. Facing them is a 100,000-strong coalition of Iraqi government forces, Kurdish fighters and Shi’ite paramilitary units, which has all but surrounded the city.

Maan, the interior ministry spokesman, said so far 955 insurgents had been killed and 108 captured on the southern frontlines alone.

He did not give a toll for the campaign overall - either for security forces, civilians or Islamic State fighters.

In an online statement, Islamic State said it killed 412 Iraqi government and Kurdish fighters during the fourth week of the battle.

On the southern front, Human Rights Watch said on Tuesday that Iraqi soldiers, militiamen and civilians had mutilated the bodies of Islamic State militants.

It said the bodies of at least five Islamic State fighters were mutilated and one militant had been executed after surrendering on Oct. 3 when the military and Sunni Muslim tribal fighters repelled an attack on Qayyara.

With air and ground support from a U.S.-led coalition, Iraqi government forces are trying to consolidate gains made in the east of the city, which they entered at the end of October.

They are yet to enter the northern or southern neighborhoods of Mosul.

Kurdish Peshmerga fighters are supporting them on the eastern and northern fronts while the Iranian-backed Shi’ite paramilitary are batting Islamic State west of the city.

More than 56,000 people have been displaced because of the fighting from villages and towns around the city to government-held areas, according to U.N. estimates.

The figure does not include the tens of thousands of people rounded up in villages around Mosul and forced to accompany Islamic State fighters to cover their retreat towards the city.