A two-lane bridge in Pittsburgh collapsed on Friday hours ahead of a visit by US President Joe Biden to the Pennsylvania city, a dramatic example of the urgency behind his drive to rebuild the United States' creaky infrastructure.

Ten people sustained injuries, all of them minor, when the snow-covered span collapsed into a wooded gully at about 6am (1100 GMT), according to authorities.

The collapse also caused a massive natural gas leak, which prompted the temporary evacuation of several families from their homes until it was brought under control, Pittsburgh Fire Chief Darryl Jones said at a news conference.

“It sounded like a snowplow,” a witness told television station KDKA, calling the timing of the collapse on the day of Biden's visit “an amazing coincidence.”

Biden was told of the bridge incident and will proceed with his trip as planned, White House press secretary Jen Psaki said. The White House was in touch with state and local officials about the collapse, she added.

The incident was a high-profile example of the need to rebuild the country's aging bridges, highways and other infrastructure with money from a $1 trillion spending bill that was a signature achievement of Biden's first year in office.

A photo posted on social media by KDKA showed several vehicles piled in the rubble of the collapsed roadway at the bottom of a wooded gully.

At least one vehicle, which appeared to be a bus, was dangling at the edge of part of the bridge. A strong smell of gas permeated the area, Pittsburgh Public Safety said in a Twitter message.

Pennsylvania has 3,198 bridges rated as being in poor condition, according to the US Department of Transportation.

Economic boost

Biden, whose approval ratings have fallen in recent months amid a surge in the Covid-19 pandemic and inflation, got a boost on Thursday when the Commerce Department reported the US economy grew the fastest in nearly four decades in 2021.

In Pittsburgh, the Democratic president will tour Mill 19, a former steel mill building now serving as a research and development hub, before hailing the US economy's strong recovery from the pandemic, the White House said.

“The president will talk about the remarkable economic progress we've made over his first year in office — including the fastest single year of job growth in American history, the biggest unemployment drop on record and, as we learned on Thursday, the fastest economic growth in 2021 in almost four decades,” a White House official said.

Biden was returning to the site of his first campaign event in 2019. He is expected to tout the creation of 367,000 manufacturing jobs since he took office a year ago, and highlight the $1 trillion infrastructure bill — which was a rare bipartisan victory in a deeply divided Congress.

The president also plans to talk about his push to rebuild American competitiveness and beat China in a race to dominate the global economy, the official said.

In recent days, General Motors Co has said it would invest $7 billion in Michigan to expand electric vehicle production and Intel Corp has said it would invest $100 billion to build a chip-making complex in Ohio.

Mill 19 is home to Carnegie Mellon University's Manufacturing Futures Institute, and hosts a robotics laboratory and a technology training site.