A major bridge in the US city of Baltimore collapsed on Tuesday (Mar 26) after being struck by a Singapore-flagged container ship, sending multiple vehicles and up to 20 people plunging into the harbour below.

A live video posted on YouTube showed a ship hitting the Francis Scott Key Bridge, after which several of its spans collapsed into the 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.

"We understand up to 20 individuals may be in the Patapsco River right now as well as multiple vehicles," Kevin Cartwright of the Baltimore Fire Department told CNN, adding officials were dealing with a "mass casualty multi-agency incident".

"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," he said.

A huge emergency response swung into action after the collision, which happened around 1.30 am local time (5.30am 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.

It opened in 1977 and carries more than 11 million vehicles a year.

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 tracking data from LSEG shows a Singapore-flagged container ship, the Dali, at the location along the Key Bridge where the accident occurred. The registered owner of the ship is Grace Ocean Pte Ltd and the manager is Synergy Marine Group, LSEG data show.

Synergy Marine Corp said that a Singapore-flagged container ship “Dali” collided with one of the pillars of the bridge and that all its crew members, including the two pilots, have been accounted for and there were no reports of any injuries.

The vessel was scheduled to sail on shipping company Maersk's route to Sri Lanka, the Danish company's website showed.

A Maersk spokesperson told Reuters the vessel was not owned by Maersk but declined to comment further.

CNA could not immediately reach Grace Ocean for comment but has contacted the Maritime Port Authority and Synergy Marine for more information.

It was not immediately clear if the Baltimore port operations were impacted due to the bridge collapse.

The port's private and public terminals handled 847,158 autos and light trucks in 2023, the most of any US port. The port also handles farm and construction machinery, sugar, gypsum and coal, according to a Maryland government website.

Baltimore Mayor Brandon Scott and Baltimore County Executive Johnny Olszewski Jr both said they were aware of the incident and that rescue efforts were underway.

"Please pray for those impacted," Olszewski posted on X.