A container ship crashed into a major bridge in the US city of Baltimore early on Tuesday, causing it to snap in several places and plunge into the river below.

Two people were pulled from the waters under the Francis Scott Key Bridge, one in serious condition, according to Baltimore fire chief James Wallace.

Several vehicles fell into the chilly waters, and rescuers were initially searching for at least seven people.

The vessel appears to have hit one of the supports of the Francis Scott Key Bridge, causing the roadway to break apart in several places and plunge into the water.

The ship caught fire and thick, black smoke billowed out of it.

Brandon Scott, mayor of Baltimore, which is in the state of Maryland, said: “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,” Mr Scott said, calling it “an unthinkable tragedy”.

Fire chief Mr Wallace said authorities “may be looking for upwards of seven people” but said that number could change. It was not clear if the two rescued were included in the seven.

Authorities said a crew of unknown size was working on the bridge at the time of the collapse and that sonar had detected cars in the water, which is about 50ft (15m) deep.

The temperature in the water was about 47F (8C) in the early hours of Tuesday, according to a buoy that collects data for the National Oceanic and Atmospheric Administration.

Earlier, Kevin Cartwright, director of communications for the Baltimore Fire Department, told The Associated Press: “This is a dire emergency.”

He called the collapse a “developing mass casualty event”, though he didn’t know at the time how many people were affected.

He added that some cargo appeared to be dangling from the bridge.

Maryland Governor Wes Moore declared a state of emergency and said he was working to get federal resources deployed.

The FBI was also on the scene.

Agencies reported receiving emergency calls around 1.30am reporting a vessel travelling outbound from Baltimore that had struck a column on the bridge, causing it to collapse.

Several vehicles were on the bridge at the time, including one the size of a tractor-trailer truck.

From a vantage point near the entrance to the bridge, jagged remnants of its steel frame were visible protruding from the water, with the on-ramp ending abruptly where the span once began.

Synergy Marine Group — which owns and manages the ship called the Dali — confirmed the vessel hit a pillar of the bridge at about 1:30am while it was in control of two pilots.