Footage shows a large section of the 1.6 mile-long Francis Scott Key Bridge falling into the Patapsco River, following the collision at around 1.30am local time (5.30am UK time).

Vehicles can be seen in the video falling from the bridge.

Earlier, Baltimore's fire service said up to 20 people were reported to be in the water, but that number has since been revised.

Baltimore bridge latest: Follow live updates

All Baltimore port traffic has been suspended until further notice, the Maryland Transport Authority, said.

In an update on Tuesday morning, James Wallace, the chief of Baltimore's fire department, said:

• Two people have already been rescued from the water;
• One of those rescued is in a "very serious condition" and the other was not injured;
• Authorities are still looking for "upwards of seven individuals" in the water;
• There is an active ongoing search and rescue operation in place over a large area;
• The crew of the ship are still on board the vessel but are communicating with the coast guard;
• Sonar devices have detected the presence of vehicles submerged in the water;

According to reports, the container ship "lost propulsion" as it left the port and the crew on board had warned Maryland officials of a possible crash, ABC News said, citing an unclassified US intelligence report.

CCTV and marine tracking data shows the Dali lose power, adjust its course and start smoking around 60 seconds before it hits the bridge.

Police commissioner, Richard Worley, said there is "absolutely no indication" that the incident was intentional or terrorism-related.

Maryland governor Wes Moore declared a state of emergency as the search for people in the water continues.

The current temperature of the water in Baltimore Harbour is between 9-10C, the National Oceanic and Atmospheric Administration in the US said.

More on the bridge collapse:
What do we know about bridge and the ship that hit it?
How did catastrophe happen?

Speaking at a news conference, Brandon Scott, Baltimore mayor, said the collapse "looked like something out of an action movie".

"Never would you think that you would see, physically see, the Key Bridge tumble down like that," he said.

Kevin Cartwright, director of communications for the Baltimore City Fire Department, called the incident "a mass-casualty, multi-agency event" that would last for many days.

He said the main focus right now was "trying to rescue and recover these people" and described the collapse as a "developing mass casualty event".

Mr Cartwright said it appears there were "some cargo or retainers hanging from the bridge," including one the size of a tractor-trailer truck.

"This is a dire emergency," he added.

The White House said it is "closely monitoring" the situation and its "hearts go out" to the families of those who are missing after the "horrific incident".

In a statement, it said: "The US Coast Guard is conducting search and rescue for those who remain unaccounted for as a result of the bridge collapse.

"Senior White House officials are in touch with the governor and mayor to offer any federal assistance they need. There is no indication of any nefarious intent."

The 289 meter-long container ship, known as the Dali, was headed to Colombo in Sri Lanka at the time of the collision.

Data from MarineTraffic showed the Singapore-flagged ship came to a halt at the bridge before two tug boats arrived at the scene.

At least 10 boats are currently involved in the rescue operation.

The same ship was previously involved in a minor incident when it hit a quay at the Port of Antwerp in Belgium in 2016, according to Vessel Finder and maritime accident site Shipwrecklog.

The vessel was reportedly damaged in the incident, but there were no injuries or pollution issues reported.

On Tuesday, Synergy Marine Group, the company that owns the container ship, said all 22 crew members, including the two pilots, have been accounted for and there were no reports of injuries.

The company said the exact cause of the incident is yet to be determined.

The ship was chartered by Danish shipping company Maersk, which said it is "horrified by what has happened". It said its thoughts are with everyone affected.

'Most likely cause is failure in machinery'

David McFarlane, director of Maritime Risk and Safety Consultants Ltd, told Sky News that the most likely cause of the incident is a "failure in machinery or steering gear" rather than human error.

He said: "The first thing that springs to my mind is: was there a sudden fault with the ship's engines or the steering gear? The other, of course, is: was there a navigational error?"

He said there "should be no room for one person errors" because of the number of other people on duty at the time.

"The most likely cause of this is a failure in machinery or steering gear, but we just won't know until the authorities have been on board. And even then, they're unlikely to say what's been going on for some considerable time," Mr McFarlane said.

Bridge is 'major artery' for locals

The bridge, which is referred to locally as the Key Bridge, was described as a "major artery" for the area by resident Michael Brown.

"This is like an April Fools' joke right now," Mr Brown told Sky News' US partner network NBC.

"It [the bridge] is a major artery in the area not only for traffic but for the port, hopefully everyone is okay that is involved."

Mr Brown said he uses the bridge to visit family and its destruction is a "major issue" - but he noted the "emphasis should be on getting whoever is in the water out".

"We have boats come through here all the time but not once have I seen a boat come close to hitting the bridge," he said.

All lanes on the Interstate 695 - the road which runs across the bridge - were closed and traffic was rerouted by the Maryland Transport Authority.

Impact on trade

Baltimore port is the 11th largest in the US, meaning its closure will cause knock-on effects for the industry.

Richard Meade, editor of Lloyd's List - which provides news on the global maritime industry - said the collision will be a "significant and expensive" operation in terms of diverting marine traffic, implications on trade and the rebuilding of the bridge itself.

According to data from MarineTraffic, around 40 ships remain inside the closed port, with a further 30 having signalled that the port was their destination.

Built in 1977, the Key Bridge is one of the longest continuous truss bridges in the world, according to the National Steel Bridge Alliance.

It was named after the writer of The Star-Spangled Banner, the national anthem of the US.

David MacKenzie, chair of engineering consultancy, COWIfonden, predicted that rebuilding the bridge will cost ten times more the approximate $60m (£47m) used to build it in the first place.

Tune into a special edition of The World with Yalda Hakim on Sky News tonight at 9pm.