A large container ship has smashed into a four-lane bridge in the US port of Baltimore, causing it to collapse and sending cars and people plunging into the river below.

Rescuers pulled out two survivors, one in very serious condition, and were searching for more in the Patapsco River after huge spans of the 2.6km Francis Scott Key Bridge crumpled into the water.

Eight people were on the bridge at the time of the collision and six remain unaccounted for, the state’s transportation secretary said hours after the collision, which closed one of the busiest ports in the United States.

“The preliminary investigation points to an accident,” Maryland Governor Wes Moore said at a briefing, and there was no credible evidence of terrorism.

When the mayday came, people stopped cars coming onto the bridge and saved lives, he said.  “These people are heroes”

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The US Coast Guard reported the collapse at 1.27am on Tuesday and it deployed crews for an active search and rescue mission after the Singapore-flagged container ship forced the trellis-like bridge up into a mangled mass of metal.

Work crews had been repairing potholes on the bridge at the time of the collapse and sonar detected vehicles under the water, which was about 15 metres deep at that point, Maryland Secretary of Transportation Paul Wiedefeld said.

The bridge was up to code and there were no known structural issues, Moore said.

He said the vessel’s crew members notified authorities of a power issue on board before the crash.

A helicopter flies over a container ship as it rests against wreckage of the Francis Scott Key Bridge.

Jayme Krause was in the midst of another night shift at her workplace on shore when the cart of packages in front of her shook violently about 2am.

A coworker told her the bridge had collapsed and she ran out to look.

“I went over there, and sure as anything, it was gone, the whole bridge was just like, there was nothing there,” she told Reuters in an interview.

“It was a shocking sight to see.”

The ship “lost propulsion” as it was leaving port, and crew on board notified Maryland officials they had lost control of the vessel, ABC News reported, citing an unclassified US intelligence report.

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Traffic was suspended at the Port of Baltimore until further notice, Maryland transportation authorities said.

It is the busiest US port for car shipments, handling more than 750,000 vehicles in 2022, according to port data.

The closure of one of the US east coast’s major ports threatens to disrupt supplies of goods from cars, to coal and other commodities like sugar.

It could create bottlenecks and increase delays and costs on the northeastern seaboard, experts say.

The port handles the most car imports and is among the largest for coal exports.

The ship was identified as the Dali, owned by Grace Ocean Pte Ltd and managed by Synergy Marine Corp.

Synergy said the Dali collided with one of the pillars of the bridge and that all its crew members, including the two pilots, had been accounted for and there were no reports of any injuries.

Baltimore Mayor Brandon Scott described a scene of twisted metal shooting into the sky.

“It was something out of an action movie. It was something you never thought you’d see,” he said.

Kevin Cartwright, the spokesman for Baltimore City Fire Department, earlier told Reuters that as many as 20 people could be in the river along with cars and possibly a tractor-trailer.

“This is a mass-casualty, multi-agency event,” he said.

“This operation is going to extend for many days.”

A live video posted on YouTube showed the ship ploughing into the bridge in darkness.

The headlights of vehicles could be seen on the bridge as it crashed into the water and the ship caught fire.

Tuesday’s disaster was the first major collapse of a bridge from a ship impact in about 40 or 50 years, David Knight, a specialist at the Institution of Civil Engineers, told Reuters.

The Dali was chartered by shipping company Maersk at the time of the incident, the Danish company said in a statement.

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

-with AAP