Officials said six of those injuries were critical, but no fatalities were reported.

An explosion was reported about 8:15 a.m., and within minutes a fire raged through the building, said Robert Ball, a spokesman for Hennepin County Emergency Medical Services. Paramedics responded to find victims on the ground, some with injuries that suggested they may have fallen multiple stories.

"It's not clear whether people were pushed out of the building from the explosion or whether they fell or jumped out of windows to escape," he said.

Authorities weren't sure whether any residents were still inside. Assistant Minneapolis Fire Chief Cherie Penn said the roof had partially collapsed, and it was too dangerous for firefighters to enter and sweep the premises.

Penn said 13 people were taken to local hospitals, and six were considered to be critically hurt.

Plumes of thick, whitish-gray smoke could be seen rising from the building, which has a grocery store on the ground floor and two levels of apartments above it. Flames could be seen through third-story windows, and the frigid air was filled was the acrid smell of smoke.

Firefighting efforts were being hampered by the sub-zero temperatures. As firefighters aimed their hoses at the flames, water gushed from windows and doorways, forming icicles on window frames and leaving the street slick and icy.

The facade of the building and trees out front were also coated with a layer of ice.

Ball said the frosty conditions were creating an additional set of hazards for the 55 firefighters on the scene.

"While heat-related illnesses are common for firefighters, now you combine that with the rapid onset of frostbite or hypothermia when they come out and they're wet and exposed to bitter cold temperatures," he said.

Outgoing Minneapolis Mayor R.T. Rybak told reporters he was "deeply concerned" about the fire.

"I'm also deeply grateful for the firefighters and other crews who came out here and kept it from getting worse," he said.

(AP)