A container ship lost power and crashed into a major bridge in the US city of Baltimore early on Tuesday, causing it to snap in several places and plunge into the river below.

The operators of the ship issued a mayday call moments before the crash that took down the bridge, enabling authorities to limit vehicle traffic on the span, Maryland’s governor said.

The ship crashed into one of the bridge’s supports, causing the structure to break apart like a toy.

Two people were rescued from the water, and officials said six people were still unaccounted for. All were believed to be those working on the bridge when it collapsed.

Maryland Bridge Collapse
The ship crashed into the major bridge in Baltimore after losing power, causing it to collapse in a matter of seconds and creating a terrifying scene as several vehicles plunged into the river below (AP Photo/Matt Rourke)

It tumbled into the water in a matter of seconds — a shocking spectacle that was captured on video and posted on social media. The vessel caught fire and thick, black smoke billowed out of it.

The crash happened long before the busy morning commute in what one official called a “developing mass casualty event”.

Brandon Scott, mayor of Baltimore, which is in the state of Maryland, said: “Never would you think that you would see, physically see, the Key Bridge tumble down like that.

US Bridge
(PA Graphics)

“It looked like something out of an action movie,” Mr Scott said, calling it “an unthinkable tragedy”.

President Joe Biden said he planned to travel to Baltimore “as quickly as I can” and that he intends for the federal government to pick up the entire cost of rebuilding the bridge.

“This is going to take some time,” Mr Biden said. “The people of Baltimore can count on us, though, to stick with them at every step of the way until the port is reopened and the bridge is rebuilt.”

Authorities said a crew of unknown size was working on the bridge at the time of the collapse and that sonar had detected cars in the water, which is about 50ft (15m) deep.

The temperature in the water was about 47F (8C) in the early hours of Tuesday, according to a buoy that collects data for the National Oceanic and Atmospheric Administration.

Maryland-Bridge-Collapse
Rescuers were searching for multiple people in the water (WJLA via AP)

Earlier, Kevin Cartwright, director of communications for the Baltimore Fire Department, told The Associated Press: that several vehicles were on the bridge at the time of the collapse, including one the size of a tractor-trailer truck.

The bridge came down in the middle of night when traffic would be lighter than during the day, when thousands of cars traverse the span.

Mr Cartwright called the collapse a “developing mass casualty event”.

File photo shows the Francis Scott Key bridge before the collapse (Alamy/PA)

Synergy Marine Group — which manages the ship, called the Dali — confirmed the vessel hit a pillar of the bridge at about 1:30am while it was in control of two pilots, who are local specialists who help navigate vessels safely into ports. The ship is owned by Grace Ocean Private Ltd.

It said all crew members, including the pilots, were accounted for and there were no reports of any injuries.

From a vantage point near the entrance to the bridge, jagged remnants of its steel frame were visible protruding from the water, with the on-ramp ending abruptly where the span once began.

Maryland Bridge Collapse
The jagged remnants of the bridge’s steel frame were visible protruding from the water (AP Photo/Steve Ruark)

Mr Cartwright said that some cargo appeared to be dangling from the bridge, which spans the Patapsco River at the entrance to a busy harbour.

The river leads to the Port of Baltimore, a major hub for shipping on the East Coast. Opened in 1977, the bridge is named after the writer of The Star-Spangled Banner, the US national anthem.

Maryland Transportation Secretary Paul Wiedefeld said all vessel traffic into and out of the port would be suspended until further notice, though the facility was still open to trucks.

Baltimore bridge incident
The bridge collapsed in the early hours of the morning (Harford County, MD Volunteer Fire & EMS)

Maryland Governor Wes Moore declared a state of emergency and said he was working to get federal resources deployed.

The FBI was on the scene, but said there was no credible information to suggest terrorism and that President Joe Biden had been briefed.

The Dali was headed from Baltimore to Colombo, Sri Lanka, and flying under a Singapore flag, according to data from Marine Traffic.

The container ship is about 985ft (300m) long and about 157ft (48m) wide, according to the website.

Danish shipping giant Maersk said it had chartered the vessel, which was carrying its customers’ cargo, though no Maersk crew and personnel were on board.

The collapse caused Maersk share at the Nasdaq Copenhagen to plummet 2% in early Tuesday trading.