The desperate search for six workers missing after the Baltimore bridge collapse has become a recovery mission, as authorities admit they are likely all dead.

More than 18 hours after the Francis Scott Key Bridge fell at 1.30am on Tuesday (local time), US Coast Guard and Maryland State Police officials said there was little – if any chance – the missing could be found alive.

It’s a “really heart-breaking conclusion to a challenging day”, Maryland Governor Wes Moore said.

“We put every single asset possible – air, land and sea” to find the missing people, he said on Tuesday night.

“Based on the length of time that we have gone on the search, the extensive search efforts that we have put in, [and] the water temperature, at this point we do not believe that we will find any of these individuals still alive,” Moore said.

“While even though we’re moving on now to a recovery mission, we’re still fully committed to making sure that we’re going to use every single asset to now bring a sense of closure to the families.”

Rescuers earlier pulled two survivors to safety. One was taken to hospital with serious injuries.

It followed the late night disaster in Baltimore Harbour when the Singapore-flagged container vessel Dali, heading out of the harbour bound for Sri Lanka, ploughed into a support pylon of the Francis Scott Key Bridge, forcing the closure of one of the busiest ports on the US eastern seaboard.

A trestled section of the of the 2.57-kilometre span almost immediately crumpled into the icy water, sending vehicles and people into the river.

The two rescued workers and the six still missing were part of a crew filling potholes on the road surface of the bridge when the accident occurred, officials said.

The ship reported a power failure and made a mayday call before impact. That gave officials crucial minutes to stop traffic on the bridge before it collapsed.

“By being able to stop cars from coming over the bridge, these people are heroes. They saved lives last night,” Moore said at a news briefing hours later.

He said the bridge was up to code and there were no known structural issues. Officials have also said there was no evidence of foul play.

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.

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.

US Transportation Secretary Pete Buttigieg said closing one of the country’s busiest shipping lanes until further notice would have a protracted impact on supply chains.

The Port of Baltimore handles more automobile cargo than any other US port – more than 750,000 vehicles in 2022, according to port data.

General Motors and Ford Motor will reroute affected shipments, but the companies said the impact would be minimal.

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 Secretary Paul Wiedefeld said.

The US Coast Guard says the six workers still missing after the collapse are presumed dead. Photo: Getty

The 289-metre Dali had experienced a momentary loss of propulsion and dropped anchors as part of emergency procedures before impact, its management company, Synergy Marine reported.

All 22 crew members aboard the vessel were accounted for, it said.

The port’s closure also threatens to disrupt supplies of goods ranging from coal to sugar.

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

President Joe Biden said the US Coast Guard responded quickly to the mayday call and commended the fast action of Maryland transportation officials who shut the bridge before it was struck and “undoubtedly saved lives”.

Biden has promised to visit Baltimore as soon as possible and said he wanted the federal government to pay to rebuild the bridge.

“I’m directing my team to move heaven and earth to reopen the port and rebuild the bridge as soon as humanly possible,” he said.

The bridge, named for the author of the Star-Spangled Banner, opened in 1977.

The US Coast Guard reported the collapse at 1.27am and it deployed search and rescue crews after the ship forced the trellis-like bridge up into a mangled mass of metal.

National Transportation Safety Board chair Jennifer Homendy said investigators were at the scene but would not board the ship while search-and-rescue operations continued.

Tuesday’s disaster might be the worst US bridge collapse since 2007, when the I-35W bridge in Minneapolis collapsed into the Mississippi River, killing 13 people.

-with AAP