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OUTSIDE MOSUL, IRAQ (CBSMiami) – The battle to re-take the Iraqi city of Mosul from ISIS control is underway with the U.S. military providing air and ground support. Defense Secretary Ashton Carter says the Mosul campaign is a “decisive moment” in the effort to defeat the Islamic State.

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Backed by a U.S. led coalition, convoys of Iraqi and Kurdish forces, plus Sunni militia men, moved in on Mosul, Iraq’s second largest city.

Just after dawn Monday, they broke through the front lines where there has been artillery fire, explosions from above and plumes of smoke on the ground.

There is also video now emerging from inside Mosul of what appears to be U.S. coalition airstrikes inside the city.

It’s thought that there are between 3 to 5-thousand ISIS fighters inside Mosul and that they have laid explosives to protect themselves.

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Monday, Iraq’s military said it inflicted “heavy losses of life and equipment” on ISIS in a district southeast of Mosul. Hours after the beginning of the offensive, Iraq’s military said it had inflicted losses and made advances in the Hamdaniya district.

Thousands of Iraqi troops face what could be their toughest battle since the withdrawal of U.S. military forces nearly five years ago.   Mosul has been under the control of Islamic state militants since June of 2014.

Isis fighters have reduced much of the city to rubble.

“Absolutely destroyed it. Very methodically, very deliberately, from one end to the other, knocked down everything, destroyed everything possible,” said Lt. Col. Christopher D. Payant of the 101st Airborne Division. The U.S. military commander in charge of the coalition said overnight the operation “may prove to be a long and tough battle, but the Iraqis have prepared for it and we will stand by them.”

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The United Nations warns as many as 1 million people in Mosul may be forced to flee the city.  The Iraqi Government has told civilians to stay inside their homes so they aren’t caught in the cross fire. Humanitarian groups have set up emergency camps nearby.