A two-lane snow-covered Bridge collapsed in Pittsburgh early Friday (Friday night AEDT), prompting rescuers to rappel nearly 46 metres while others formed a human chain to help rescue multiple people from a dangling bus.

The collapse came hours before US President Joe Biden was set to visit the city on a pre-planned trip to press for his $1 trillion infrastructure bill, which includes bridge maintenance.

There were minor injuries from the collapse but no fatalities, said authorities, who also said they were flying drones to make sure no one is under any collapsed sections.

Fire Chief Darryl Jones said there had been 10 "minor injuries" and three people were transported from the scene. None of the injuries were life-threatening, he said. Three or four vehicles were on the bridge when it fell, Jones said.

A strong smell of natural gas has been reported in the area, police said, later adding that a gas line had been cut in the area.

The Red Cross has been contacted to assist victims, and residents are asked to avoid the area.

The Allegheny County Police Department was notified about a partial bridge collapse sometime after 6am on Friday (10pm AEDT), according to Amie Downs, a spokesperson for the county.

Four vehicles were on the bridge at the time of the collapse, Downs said, and only minor injuries had been reported, among them the driver of an articulated bus and two passengers.

Jeremy Habowski told CNN he feels lucky to be alive after surviving the collapse. He and other drivers were unable to stop in time after the bridge gave way, he said.

"The scariest part was definitely going over the edge because there was a gap and my car left the ground," he told CNN. His car and others are now sitting on top of what remains of the bridge.

Once things settled, Habowski checked on another driver, he said. Then he climbed up the hill to try and stop other drivers from driving into the debris.

"It was a lot to take in," he told CNN.

A photo from the scene showed a commuter bus upright on a section of the collapsed bridge.

Biden is aware of the collapse, the White House said in a statement, and plans to proceed with his trip.

"The President is grateful to the first responders who rushed to assist the drivers who were on the bridge at the time," the statement reads.

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