American-backed forces in Iraq continue to battle ISIS in western Mosul over the weekend. Since late last month, more than 45,000 refugees are reported to have fled the city. Iraqi forces said the past 24 hours have seen the heaviest fighting yet in the battle to retake Mosul. As they push deeper into heavily populated areas, civilians are paying the price.

They’ve lost everything in the desperate race to get away, including the shoes on their feet, reports CBS News correspondent Charlie D’Agata.

One man said it was raining hard and ISIS opened fire as they tried to escape. As they ran away, they lost their shoes in the mud. Freezing, pouring rain has only added to the misery of tens of thousands now made homeless.

The heavy cloud-cover brought with the harsh weather has also complicated air support for U.S.-backed Iraqi forces, making it much harder to detect and destroy ISIS car bombs.

As Iraqi forces fight it in the congested neighborhoods of western Mosul, ISIS militants are battling back with everything they’ve got -- including suspected chemical weapons.

The U.N. is calling for an investigation into a possible chemical weapons attack after 12 people, including women and children, were treated at a hospital in Erbil for suspected exposure to what the Red Cross described as “blistering agents.”

“You can see on their bodies were extreme bodily reactions, for example there are blisters. A lot of them came coughing, vomiting, redness in the eye,” said Sara al-Zawqari, spokeswoman for the International Committee of the Red Cross in Iraq.

It took months for Iraqi forces to drive ISIS out of eastern Mosul, all the way up to the Tigris River which carves the city in two. The densely-populated western side is where ISIS leader Abu Bakr al-Baghdadi declared himself the leader of the so-called caliphate in July 2014.

ISIS has vowed to defend it to the death. It’s never mattered how many civilians may stand in the way.

A victory in Mosul may deal a final blow to ISIS in Iraq. It was the biggest city under ISIS control, and losing it will mean the group no longer holds territory in that country in any meaningful way.

Meanwhile, in Syria, militias backed by the U.S. have reportedly cut a key road out of Raqqa, the de facto ISIS capital in Syria. The city is now surrounded on three sides, and the only way out for ISIS fighters is across the Euphrates River.