Battle for Mosul

A general said poor visibility was limiting the ability of aircraft to provide cover, adding that troops would secure eastern areas they had entered.

IS militants were putting up fierce resistance, using snipers, booby traps and suicide car bombs.

Seven civilians were reportedly killed by mortar fire in one recaptured area.

Mosul, which was captured by IS in June 2014, is the jihadist group's last major urban stronghold in Iraq.

Troops came within hundreds of meters of Mosul on Monday evening and are now the closest they've been since launching an operation two weeks ago to wrest the city from more than two years of ISIS rule.

Experts and officials have said that entering Mosul will likely trigger the fiercest fighting seen yet in the offensive and that the battle is expected to be fought street to street or even house to house.

Yet at dusk on Saturday, the 2 kilometers between the US-trained Golden Division and the medieval world of the so-called caliphate suddenly were aflame with the loathing and terror of the world's war on ISIS.

Tracer rounds flashed across the horizon; the sparks and thud of countless explosions rocked the tiny village of Bazwaya, split in two by the closing stages of this battle. Ferocious and constant, it came closer and closer to the Iraqi base where we filmed from a rooftop.

But for all its gains -- 78 villages liberated, and nearly 800 ISIS fighters killed as of Monday morning -- the Iraqi coalition is encountering fierce resistance from ISIS, in what is anticipated to be the terror group's last stand in the country.

Massively outnumbered by the advancing coalition -- a 90,000-strong force of Iraqi government troops, Kurdish Peshmerga fighters and irregular militia soldiers -- ISIS relies on asymmetric warfare tactics to inflict damage on its opponents, and terrorize long-suffering civilian populations.

A coalition of 100,000 troops have been closing in on Mosul since Monday, liberating surrounding communities village by village and making quicker-than-expected gains.

The coalition vastly outnumbers its opponent. No more than 5,000 ISIS fighters are in Mosul, a US military official said, although the terror group's supporters put the number at 7,000.

Officials and analysts say that entering Mosul is likely to kick off intense street fighting as coalition forces try to retake what has become the cultural capital of ISIS' envisaged caliphate, or Islamic state.

Those killed on Thursday and Friday had been rounded up near and in the city for use as human shields against attacks that are forcing ISIS out of the southern sections of Mosul, the source explained.

ISIS used a bulldozer to dump the corpses in a mass grave at the scene of the executions -- Mosul's defunct College of Agriculture in the north of the city, the intelligence source said.

The victims were all shot and some were children, said the source, who wanted anonymity because he is not authorized to speak to the media. CNN could not independently confirm the claim.

Twelve IS jihadists were also killed in the dawn attack.

Hours later, witnesses said gunfire was still being heard and militants were walking openly through the streets.

Separately, Iraqi pro-government forces are continuing an offensive to re-take the IS-held city of Mosul to the south.

A news agency affiliated to IS claimed its fighters had broken into Kirkuk's town hall and seized a central hotel, but officials denied this.

Artillery began firing on the city early on Monday, in a long-awaited assault from Kurdish Peshmerga, Iraqi government and allied forces.

Tanks are now moving towards the city, which has been held by IS since 2014.

The UN has expressed "extreme concern" for the safety of up to 1.5 million people in the area.

The BBC's Orla Guerin, who is with Kurdish forces east of Mosul, says tanks are advancing on the city, kicking up clouds of dust.