A major bridge in the US city of Baltimore collapsed early on Tuesday after being struck by a container ship, sending multiple vehicles and people plunging into the frigid harbour below.

Dramatic footage showed a 300-metre vessel hitting the Francis Scott Key Bridge, bringing the steel-built structure crashing into the Patapsco River.

Lights from what appear to be vehicles could be seen on the road surface as the bridge warped and crashed in sections, with the third tranche cantilevering upwards before it, too, tumbled into the water.

As daylight broke over the search and rescue operation with divers in the water looking for survivors, the horrifying extent of the incident became apparent.

Twisted stanchions of steel lay draped over the deck of the ship, on which stacks of containers teetered precariously — adding a further dimension of danger to rescue and recovery work.

Baltimore Mayor Brandon Scott said the incident was an “unthinkable tragedy,” adding “we have to be thinking about the families and people impacted, folks who we have to try to find.”

He described images of the collision as “like something out of an action movie.”

The footage appeared to show the ship going dark twice in the moments before the collision, possibly indicating some kind of power failure on board.

There was no immediate confirmation of the cause of the disaster, but Baltimore’s Police Commissioner Richard Worley said there was “no indication” of terrorism.

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

Water temperatures were around 9 Celsius, narrowing the window of survivability.

Baltimore’s fire chief James Wallace said sonar had “detected the presence of vehicles” in the harbour, but declined to estimate how many.

One person was taken to hospital in “a very serious condition,” he said, adding a second person recovered from the water was uninjured.

“We may be looking for upwards of seven individuals,” he said.

‘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,” he said.

The 2.6-kilometre, 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.

Maryland Governor Wes Moore declared a state of emergency, a move that will ease the release of federal resources.

Shipping giant Maersk said the “DALI,” which was en route from Baltimore to Colombo in Sri Lanka, was being operated by charter vessel company Synergy Group, and carrying cargo belonging to Maersk customers.

“We are horrified by what has happened in Baltimore, and our thoughts are with all of those affected,” a Maersk statement said.

“We are closely following the investigations conducted by authorities and Synergy, and we will do our utmost to keep our customers informed,” it said.

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

“Rescue efforts remain underway and drivers in the Baltimore area should follow local responder guidance on detours and response,” US Secretary of Transportation Pete Buttigieg said on social media.

Baltimore is home to one of the busiest ports in the United States, handling around $80 billion worth of cargo last year, including a large volume of vehicles.

If the bridge’s collapse hampers access to the Port of Baltimore, it could have a significant economic impact.