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Thick sheets of ice left on a Chicago warehouse are threatening to collapse the building after firefighters twice had to beat back flames there during a frigid cold spell.

The abandoned warehouse at the intersection of 37th and Ashland in Chicago's Bridgeport neighborhood burst into flames on Tuesday night. Firefighters succeeded in putting out the blaze - one of the city's biggest in decades - but it reignited a little more than a day later.

All the water poured onto the building meant more ice on the structure because of the frigid temperatures that have hit the city in recent days. All the ice has put more weight on the five-story warehouse.

"The last few days have been really tough. It's been really cold and really wet," Peter Vandorpe of the Chicago Fire Department told ABC News' Alex Perez.

"This is one of the most dangerous stages of the operation," he said. "The building is weak. The building has a lot of ice on it. The fire is still burning in areas that we can't get to. Everybody's tired. Everybody's cold. And part of it ... is there isn't a lot of activity here. So the less active you are, the colder you get. You get numb, you get cold, it's hard to concentrate and that makes it dangerous. It doesn't look dangerous, but it is."

Vandorpe said frostbite was a key concern for him and his crew.

"We're standing out here in the middle of the day, we're not completely covered, and you can do that for a little while, but it'll sneak up on you," he said. "You'll get frostbite on your ears, your nose, your lips, places like that before you realize it. So we've got to keep an eye on our guys. Otherwise, you know, they won't be paying attention. They're focused on the task that they're doing.

"It's not what you can take. It's not about how tough you are," he said. "It's what your body can take, parts that you can't monitor or you can't pay attention to.

"Slips increase," he added, "falls increase, the frostbite increases, your body has to work hard just to keep yourself warm, so you're going to get fatigued faster."

When the fire first broke out on Tuesday, approximately 50 fire companies and 170 firefighters reportedly headed to the scene, about a third of the city's fire department.

The cause of the fire is now under investigation. No injuries have been reported.

Chicago has been hit by the bitterly cold weather sweeping the Midwest in the past few days. The temperature has been below freezing since Tuesday, with wind chills as low as 16 below zero.