A container ship has rammed into a major bridge in Baltimore, Maryland, causing it to snap and plunge into the river below.

Several vehicles fell into the chilly waters of the Patapsco River early on Tuesday, and rescuers searched for survivors.

It was not clear what caused the cargo ship to crash into the Francis Scott Key Bridge long before the busy morning commute in the United States city close to Washington, DC.

Two people were rescued, and it was not clear how many more might be in the waters of the busy harbour near a key port.

The ship crashed into one of the bridge’s supports, causing the structure to snap and buckle at several points and tumble into the water in a matter of seconds — a shocking spectacle that was captured on video and posted on social media. The vessel caught fire, and thick, black smoke billowed out of it.

“Never would you think that you would see, physically see, the Key Bridge tumble down like that. It looked like something out of an action movie,” Baltimore Mayor Brandon Scott said, calling it “an unthinkable tragedy”.

Baltimore Fire Department Chief James Wallace said authorities “may be looking for upwards of seven people”, but he said that number could change, and other officials wouldn’t give figures. It was not clear if the two people rescued were included in the seven cited by the fire chief.

Authorities said a crew of unknown size was working on the bridge at the time of the accident and sonar had detected cars in the water, which is about 15 metres (50ft) deep. The water temperature was about 47 degrees Fahrenheit (8 degrees Celsius) before dawn on Tuesday, according to a buoy that collects data for the National Oceanic and Atmospheric Administration.