A major bridge in the US city of Baltimore almost entirely collapsed Tuesday after being struck by a cargo ship, sending multiple vehicles and up to 20 people plunging into the harbor below.

Dramatic footage shows a container ship hitting a footing of the Francis Scott Key Bridge, sending the steel-built structure into the Patapsco River.

Lights from what appear to be vehicles can be seen on the road surface as the bridge warps and crashes in sections, with the third tranche cantilevering upwards before it, too, tumbles into the water.

Several small explosions can also be seen.

"Unfortunately, we understand that there were up to 20 individuals who may be in the Patapsco River right now as well as multiple vehicles," Kevin Cartwright of the Baltimore Fire Department told CNN.

"So we have... a mass-casualty multi agency incident. underway.

"We understand that there could have potentially been a vehicle, a tractor trailer or a vehicle as large as a tractor trailer on the bridge at the time that it collapsed."

A huge emergency response swung into action after the collision, which happened around 1:30 am (0530 GMT), with first response vehicles crowding the shoreline.

Divers were in the water looking for survivors, Cartwright said, with temperatures hovering around freezing.

Photographs from the scene show debris from the bridge resting on the deck of the ship, where containers were stacked several high.

Reports suggested some of the containers were unstable, complicating rescue efforts.

"This is a very devastating incident which these individuals have encountered so our hearts go out to each and every one of them and we will be working valiantly throughout the early morning and day to address this," said Cartwright.

The 1.6-mile (2.6-kilometer), four-lane bridge spans the Patapsco River southwest of municipal Baltimore.

It opened in 1977 and carries more than 11 million vehicles a year, around 31,000 a day.

It is a major part of the road network around Baltimore, an industrial city on the US East Coast next to the capital Washington DC.

The Maryland Transportation Authority told drivers to avoid the bridge, part of the I-695 interstate highway, which it called an "active scene".

Ship monitoring website MarineTraffic showed a Singapore-flagged container ship called the Dali stopped under the bridge early Tuesday.

Logs show the 300-meter vessel was en route from Baltimore to Colombo in Sri Lanka.

Dali's registered owner Grace Ocean Private Ltd could not be reached for comment.

Baltimore is one of the busiest cargo ports in the United States, handling billions of dollars of vehicles, containers, forest products and project cargo, according to the city's port authority.