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

Dramatic footage showed a 300-meter vessel hitting a footing of the Francis Scott Key Bridge, sending the steel-built structure crashing 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 during the rapid collapse.

"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."

The footage appeared to show the ship going dark twice in the moments before the collision, with thick black smoke issuing from the ship's funnel, possibly indicating activity from the engine.

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.

CNN reported that water temperatures were around 48 Fahrenheit (8.8 Celsius), complicating search and rescue efforts and narrowing the window of survivability for anyone in the water.

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.

- 'Sound of thunder' -

A man who identified himself as a former Baltimore fire department worker said he could see the bridge from his bedroom window.

"We were awakened by what appeared to be an earthquake and a long, rolling sound of thunder," he told local media.

"I saw some emergency lights in the area and decided to drive up... what was in progress was a multi-jurisdictional response to a disaster."

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

The Maryland Transportation Authority told drivers to avoid the area, 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 vessel was en route from Baltimore to Colombo in Sri Lanka.

The BBC reported Synergy Marine Group confirming its vessel had been involved.

"Whilst the exact cause of the incident is yet to be determined, the 'Dali' has now mobilised its Qualified Individual Incident response service," it says.

All crew, as well as pilots -- the specialised mariners who navigate vessels around port areas -- have been accounted with no reports of any injuries.

US Secretary of Transportation Pete Buttigieg said on social media that he was in contact with the mayor of Baltimore and the governor of Maryland, and had offered federal help.

"Rescue efforts remain underway and drivers in the Baltimore area should follow local responder guidance on detours and response," he wrote on X, formerly known as Twitter.

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.