The collapse happened hours before President Joe Biden's scheduled visit to the Pennsylvania city to promote his infrastructure plan.
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Three or four vehicles were on the two-lane commuter bridge, which spans over a creek in Frick Park, when it collapsed Friday morning, Pittsburgh Fire Chief Darryl Jones said. Ten people suffered non-life-threatening injuries, and three were transported to nearby hospitals.
Images from the scene show a bus atop a section of the bridge that fell and sloped upward at a near 45-degree angle. Officials said only a driver and passenger were aboard the bus at the time. There was also a car upside down in front of the bus.
First responders trying to help in the icy conditions were among the injured, Jones said. Responders rappelled down about 150 feet to reach the collapse site, as others formed the human chain, the Associated Press reported.
Pennsylvania Lt. Gov. John Fetterman called the bridge "a vital infrastructure artery" for the city, adding, "It's surreal this morning to see it completely collapsed."
"Thank God that school buses were delayed due to weather, so there was less traffic than normal. Thank God there have been no casualties reported at this point. I wish all of those who were injured a safe and swift recovery," Fetterman said.
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A strong natural gas odor was detected after the collapse, and a gas line has since been cut, authorities said. Some residents were evacuated from their homes.
Residents are advised to avoid the area.
The cause of the collapse is under investigation. The bridge was last inspected in September 2021, officials said.
The National Transportation Safety Board is sending a team to the scene.
Biden is aware of the collapse and "is grateful to the first responders who rushed to assist the drivers who were on the bridge at the time," according to a White House statement. He will continue Friday's trip as planned, White House officials said.
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The president is slated to tout his Bipartisan Infrastructure Law, which is set to provide $1.63 billion to Pennsylvania in federal funding for bridges alone, the third highest figure for any state. Pennsylvania has 3,353 bridges in poor condition, the second most after Iowa, according to data from the administration.
Local officials, however, will be authorized to allocate funding to bridges.
ABC News contributed to this report.