Rescue teams are searching the water under the bridge for casualties, with as many as 20 workers thought to have fallen in, according to the Baltimore City Fire Department.

Here's everything we know about the incident so far.

What happened?

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

The city fire department's communications chief Kevin Cartwright has called the incident a "developing mass casualty event", adding it was too early to know how many people were affected.

What does the video show?

It shows the vessel approaching the bridge before the structure collapses into the water.

Vehicles can be seen falling from the bridge.

What do we know about the cars that fell into the river?

Mr Cartwright said multiple vehicles were on the Key Bridge when it was struck by the ship, one of which was the size of a tractor-trailer.

He said it appears there are "some cargo or retainers hanging from the bridge", creating unsafe and unstable conditions, and that emergency service workers were operating cautiously as a result.

All lanes were closed before bridge collapse

All lanes were closed on the bridge before the collapse and all traffic was being re-routed, the Maryland Transport Authority said just before 2am.

What do we know about the bridge itself?

The Francis Scott Key Bridge is an enormous steel structure which carries the Interstate 695 highway over the Patapsco River southeast of the Baltimore metropolitan area.

Built in 1977 and referred to locally as the Key Bridge, it was named after the author of the American national anthem.

The bridge is more than 8,500ft, or 1.2 miles, long in total.

Its main section spans 1,200ft and was one of the longest continuous truss bridges in the world upon its completion, according to the National Steel Bridge Alliance.

What do we know about the ship that hit it?

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

It appears to have sustained damage, a fire department official told Sky News partner network CNBC, though the ship's management company Synergy Marine Group said all crew on board have been accounted for and no injuries have been reported.

It confirmed the Singapore-flagged Dali vessel collided with one of the Key Bridge pillars while under the control of two pilots. The firm said the exact cause of the incident was yet to be determined.

The vessel's individual incident response service has been mobilised, Synergy added.

According to ship tracking website VesselFinder, Dali had been involved in a 2016 collision at the Port of Antwerp as it tried to leave the port.

It said the vessel had "substantial stern momentum" causing the collision with a stone wall of the quay.

The berth where the ship had been docked was "seriously damaged and closed for cargo handling operations due to safety reasons", the website said.

What have experts said?

A maritime safety expert says the "most likely" cause of the crash was a failure in the ship's machinery.

David McFarlane, director of Maritime Risk and Safety Consultants Ltd, told Sky News: "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?"

Mr McFarlane said a human error was less likely because of the number of people who would have been on duty.

He added: "There should be no room for one-person errors because one of the other people should jump in and say 'hang on...'.

"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."