CHICAGO - An explosion at a building in Chicago on Tuesday injured at least six people, including three who were in a serious or critical condition, the city's fire department said.

Images and a video clip posted by the fire department showed damaged top floors of a red-brick building at Chicago's West Washington Boulevard and North Central Avenue.

The video showed bricks scattered around the corner building and parked cars damaged from fallen debris.

The cause of the explosion, which prompted the evacuation of an adjacent building, was not known, the Chicago Fire Department said on Twitter.

Chicago's bomb squad as well as officers from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) were on the scene, the fire department said.

The fire department was searching the collapsed structure for trapped people and at least 10 ambulances were at the site of the explosion. REUTERS