WASHINGTON (AFP) - Two parachutists were injured on Saturday after apparently colliding in mid-air while performing a stunt at an air show in Chicago, the United States military said.

One of the parachutists made a "hard landing" as he hit a building at the Chicago Air & Water Show, Pentagon spokesman Peter Cook said.

"Two parachutists, one may have been knocked unconscious or something in mid-air so he had a hard landing and hit a building," Mr Cook said.

Ms Donna Dixon, a spokesman for the US Army's Golden Knights, told local NBC affiliate WMAQ-TV that her team's parachutist was in critical condition and underwent surgery.

The other parachutist, from the US Navy's Leap Frogs, broke his leg, the television station said.

Ms Dixon said the men had been performing a stunt known as a "bomb burst", during which they circle each other in the air while separated by red smoke. They collided during the movement.

Eyewitness Heather Mendenhall said one of the parachutists caught the roof of a nearby building with his feet and fell over, his parachute trailing behind him.

"It looked like he was going to land on our building. We saw him actually, his legs clipped the top of our building and he fell to the ground," she said.

"He didn't look like he was conscious... He looked like a dummy, actually, because he didn't look like he was awake."