Aerial photo shows a Port Authority bus that was on a bridge when it collapsed on Friday, is visible through trees in Pittsburgh's East End. AP

Pittsburgh's public safety authorities tweeted that three people were hospitalised with non-life-threatening injuries after the road bridge buckled into a snowy ravine.

The otherwise minor accident immediately caught national attention because Biden was set to touch down in the industrial city shortly for a speech promoting his efforts to reset the post-pandemic US economy, including through the historic infrastructure spending splurge.

Joe Biden boards Air Force One at Joint Base Andrews in Maryland on Friday. AFP

Biden "has been told of the bridge collapse in Pittsburgh," Press Secretary Jen Psaki tweeted, and will "proceed with (the) trip planned for today and will stay in touch with officials on the ground about additional assistance we can provide."

While in Pittsburgh, located in the political battleground state of Pennsylvania, Biden was to tour Mill 19. The former mill dates back to 1943 and once churned out more than a million tons of metal a year.

Today, the site is being held up as a symbol of what the White House calls Biden's "vision to rebuild America's economy for the 21st century." Home to Carnegie Mellon University's Manufacturing Futures Institute (MFI), Mill 19 focuses on high-tech research and development.

Vehicles including a Port Authority bus are left stranded after a bridge collapsed in Pittsburgh. AFP

"The president will talk about how his bipartisan infrastructure law is already strengthening in our supply chains and critical infrastructure — our roads, bridges, ports, airports and more — giving us an edge in producing more in America and exporting it to the world," a White House official said.

Biden's political woes

For Pittsburgh's mayor, Ed Gainey, the Biden visit was welcome — a chance to home in on the kinds of problems plaguing post-industrial cities across the country, where bridges, highways, water pipes and other basic infrastructure typically dates back multiple decades.

Vehicles that were on a bridge when it collapsed are visible in Pittsburgh. AP

"This is critical that we get this funding and we're glad to have the president coming today," he told CNN. In a tough first year in office, the infrastructure bill, passed with rare cross-party Republican support, was one of Biden's biggest successes.

For years, presidents had failed to get Congress to revamp the sector, with Donald Trump's repeated promises of "infrastructure week" turning into a running Washington joke. But Biden has faced heavy setbacks on other priorities, most recently his attempt to get new voting rights guarantees through Congress. He is also embroiled in the standoff with Russia over Ukraine.

Despite signs of a roaring economic comeback from the Covid-19 shutdown, the recovery is proving uneven and inflation is eating into wage increases.

Agence France-Presse