A two-lane bridge in Pittsburgh has collapsed hours ahead of a visit by US President Joe Biden to the Pennsylvania city, dramatically underscoring the urgency behind his drive to rebuild the country's creaky infrastructure.
Biden told reporters after landing near Pittsburgh on Friday afternoon that he planned to go the site of the collapse.
After the snow-covered span collapsed into a ravine about 6am, rescue workers rappelled at least 46 metres into the gully and used ropes to help pull people to safety, Pittsburgh Fire Chief Darryl Jones said.
"They helped the firefighters that were here initially on scene, also did like a daisy chain, with hands just grabbing people and pulling them up," Jones said.
Ten people suffered minor injuries, including four who were taken to the hospital, city officials said.
Jones added that crews planned to search under the Forbes Avenue bridge for any victims.
The collapse also caused a massive natural gas leak that forced the evacuation of several homes before being brought under control, Jones said.
The US National Transportation Safety Board said it was sending a team to the site.
The incident was a high-profile example of the need to rebuild the country's ageing bridges, highways and other infrastructure with money from a $US1 trillion ($A1.4 trillion) spending bill that was a signature achievement of Biden's first year in office.
Images of the collapse showed the span buckled into three large sections, with several vehicles piled in the rubble of the collapsed roadway at the bottom of the ravine.
The tail end of a long, red city bus appeared trapped by the rubble.
Pennsylvania has 3198 bridges rated as being in poor condition, according to the US Department of Transportation.
The White House said Biden will on Friday tour Mill 19, a former steel mill building in Pittsburgh now serving as a research and development hub, before hailing the US economy's strong recovery from the pandemic.