A bridge built in 1970 has collapsed in the American city of Pittsburgh, Pennsylvania, just hours before the president was scheduled to visit to talk about infrastructure.

Four vehicles, including a bus, were on the steel-framed structure when it snapped and fell into a creek and caused a natural gas line to break and leak.

Rescuers had to rappel nearly 50 metres to rescue drivers and passengers while others formed a human chain to reach multiple people from a dangling bus.

Pittsburgh Bureau of Fire Chief Darryl Jones said there were 10 minor injuries, with three brought to the hospital, but no fatalities and authorities said the gas line was turned off.

The collapse came hours before President Joe Biden was to visit the city to press for his $US1 trillion ($1.4 trillion) infrastructure bill, which includes bridge maintenance.

The bill sets aside some $US40 billion ($57 billion) for bridge repair, replacement and rehabilitation which the White House says would be the largest such investment since the construction of the interstate highway system in the 1950s.

About 45,000 American bridges are in poor condition, according to the White House.

The bridge through one of Pittsburgh’s biggest parks travels over a creek. Photo: Getty

The 135m bridge was in Frick Park, one of the city’s biggest green spaces on Forbes Avenue over Fern Hollow Creek.

Authorities said about 15,000 cars a day used the ‘vital artery’ and it was lucky the destruction did not happen one hour later during the rush hour.

Jeremy Habowski told CNN he felt lucky to be alive as he and other drivers were unable to stop in time.

“The scariest part was definitely going over the edge because there was a gap and my car left the ground,” he told CNN, describing how he checked on another driver then climbed up the hill to try and stop others.

“It was a lot to take in.”

US President Joe Biden speaks with first responders during a visit to the scene. Photo: Getty

Pittsburgh Mayor Ed Gainey said the bridge was last inspected in September 2021 and was not considered a problem.

“If there was any warning or concern, we would have been notified and we would have made sure that we didn’t use this route,” he told CNN.

“So this one caught everybody by surprise this morning.”

Frick Park is one of the largest in Pittsburgh. Photo: Google Maps

Mr Biden proceeded with his planned trip to Pittsburgh and visited the scene where he spoke to first responders.

“Our team is in touch with state and local officials on the ground as they continue to gather information about the cause of the collapse,” a White House statement said.

“The president is grateful to the first responders who rushed to assist the drivers who were on the bridge at the time.”

-with AAP