With dive teams facing increasingly treacherous conditions in the darkened, wreckage-strewn waters, active search-and-rescue operations were suspended about 18 hours after the accident, US Coast Guard and Maryland State Police officials said.

Coast Guard Rear Admiral Shannon Gilreath said there was no hope of finding the missing workers alive due to the frigid water and the length of time elapsed since the accident.

State Police Colonel Roland Butler said authorities hoped to return divers to the water after sunrise on Wednesday in an effort to recover the workers' remains.

The Singapore-flagged container vessel named Dali, heading out of Baltimore Harbor bound for Sri Lanka, ploughed into a support pylon of the Francis Scott Key Bridge over the mouth of the Patapsco River about 1.30am.

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

Rescuers pulled two survivors to safety, one of whom was hospitalised.

They and the six missing were part of a work crew filling potholes on the bridge.

The ship reported a power failure before impact, which enabled officials to stop traffic on the bridge before the collapse.

"By being able to stop cars from coming over the bridge, these people are heroes. They saved lives last night," Maryland Governor Wes Moore said at a midday news briefing.

The bridge was up to code with no known structural issues, Moore said.

There was no evidence of foul play, officials said.

Video on social media showed the vessel slamming into the bridge in darkness, the headlights of vehicles visible on the span as it crashed into the water and the ship caught fire.

US Transportation Secretary Pete Buttigieg said closure of one of the country's busiest shipping lanes until further notice would have a "major and protracted impact to 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 disruption would be minimal.

The 289m vessel had experienced a momentary loss of propulsion and dropped anchors as part of emergency procedures before impact, management company Synergy Marine said.

The Dali, owned by Grace Ocean, rammed into one of the pillars of the bridge.

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

Besides impacts to auto shipments, the port closure could force shippers to divert Baltimore-bound cargo from containers to bulk material.

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 closed the bridge before it was struck.

Biden 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," Biden said.

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

National Transportation Safety Board chair Jennifer Homendy said investigators were at the scene but would not board the ship during search operations.

The bridge serves as the main thoroughfare for motorists between New York and Washington seeking to avoid downtown Baltimore.

The Dali was chartered by Danish shipping company Maersk, which said it was "horrified by what has happened in Baltimore and our thoughts are with all of those affected".

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.

More than 40 ships remained inside Baltimore port including small cargo ships, tug boats and pleasure craft, MarineTraffic data showed.

Australian Associated Press