Baltimore bridge rescue effort continues after container ship crash ...

Rescuers are still searching for six roadworkers who were on a bridge that was brought down by an out-of-control container ship in the US city of Baltimore.

But authorities say many lives were saved by a mayday call from the ship's crew, which prompted local officials to stop traffic driving onto the bridge before it collapsed into the river below.

Here's the latest information about the bridge disaster and the ongoing search-and-rescue operation.

What happened?

About 1:30am local time (4:30pm AEDT), a container ship ran into a pylon of the Francis Scott Key Bridge, sending it crashing into the Patapsco River below.

Eight people are known to have been on the bridge at the time.

Two of them were rescued. One was okay, but the other was seriously injured and taken to hospital. The other six people are unaccounted-for.

The ship was carrying cargo destined for Sri Lanka.(Reuters: ABC affiliate WJLA)

The ship's crew issued a mayday call before it hit the bridge, alerting the Maryland Department of Transportation that they had lost control of the ship.

That gave authorities time to stop traffic from driving across the bridge, Maryland Governor Wes Moore said.

"By being able to stop cars from coming over the bridge, these people are heroes," he said. "They saved lives."

In police radio audio published online, officers can be heard talking about the need to stop traffic as the ship approaches.

"Hold all traffic on the Key Bridge," a man says. "There's a ship approaching that's just lost their steering, so until you get that under control we've gotta stop all traffic."

The conversation turns to the crew of roadworkers on the bridge. "You might want to notify whoever the foreman is, see if we can get them off the bridge temporarily."

Another voice is later heard saying: "The whole bridge just fell down."

Who was on the bridge?

The eight people on the bridge at the time were members of a roadwork crew "basically repairing potholes", local transport department secretary Paul Wiedefeld said.

A search and rescue effort is ongoing.

President Joe Biden said: "Our prayers are with everyone involved in this terrible accident and all the families, especially those waiting for the news of their loved one right now.

"I know every minute in that circumstance feels like a lifetime."

A satellite image shows a bird's-eye view of the container ship and collapsed bridge.(Reuters: Maxar Technologies/Handout)

Initially, authorities said up to 20 vehicles also plunged into the river.

"I can tell you our sonar has detected the presence of vehicles submerged in the water," local fire chief James Wallace told an early morning press conference. He added that rescue divers were searching the river for survivors in challenging conditions.

Mr Wallace told CNN that three passenger vehicles, a cement truck and another vehicle were found in the water using infrared and sonar technology.

But at a later press conference, when asked if anyone was likely to be in the submerged vehicles, Mr Wiedefeld said, "No, we do not believe so."

The Francis Scott Key Bridge was one of three river crossings.(Reuters: Julia Nikhinson)

What caused the ship to crash?

Authorities say that's still being investigated by local authorities and the ship's operators. But the Maryland governor said the preliminary investigation "points to an accident".

"We haven't seen any credible evidence of a terrorist attack," Mr Moore said.

He said the ship's crew notified authorities they were having "a power issue" before the crash.

What else is known about the ship?

The ship was the Singapore-flagged container vessel Dali, its operators Synergy Group confirmed.

The charter vessel company said there were 22 crew members on board at the time, all of whom were Indian.

"All crew members, including the two pilots have been accounted for and there are no reports of any injuries," Synergy said in a statement. "There has also been no pollution."

Dali is owned by Grace Ocean Private Ltd, which is also based in Singapore.

It had been chartered by Danish shipping giant Maersk. "We are horrified by what has happened in Baltimore, and our thoughts are with all of those affected," a Maersk statement said.

A view of the Singapore-flagged container ship Dali after it collided with a pillar of the Francis Scott Key Bridge.(Reuters:  Harford County MD Fire & EMS)

The ship was stacked with containers and was bound for Colombo, Sri Lanka.

Dali was also involved in an accident in Antwerp, Belgium, in 2016. The ship sustained damage to its hull, and was reportedly detained by authorities, after scraping the side of a quay after leaving a container terminal.

What is the Francis Scott Key Bridge?

The steel truss bridge is part of the interstate highway I-695, also known as the Baltimore Beltway.

It's more than 2.5 kilometres long, and is one of three major river crossings at Baltimore Harbour.

"This is a place that is a normal commute route for over 30,000 Marylanders every single day," Mr Moore, the state governor, said.

"And so to hear the words that the Key Bridge has collapsed, it's shocking and heartbreaking."

Wreckage remains submerged in the Patapsco River.(AP: Mark Schiefelbein)

The bridge opened in 1977.

It was named after the author of America's national anthem, Francis Scott Key, who penned Star Spangled Banner near the Patapsco River in 1814.

Joe Biden said he hoped the federal government would cover the full cost of rebuilding it.

"I expect the Congress to support my effort," he said.

Are there implications for shipping?

Ship traffic has been suspended in the Port of Baltimore, which is one of the largest shipping hubs in the US.

According to the Maryland state government, it handles the more cars, light trucks, and "roll on-roll off" farm and construction machinery than any other US port.

A helicopter flies over the scene of the Francis Scott Key Bridge collapse in Baltimore. (Reuters: Julia Nikhinson)

"The port here in Baltimore does the most vehicle handling of any port at all," US Transportation secretary Pete Buttigieg said. "And that's just talking about the vehicle side – you also have container traffic, you have bulk traffic.

"There is no question that this will be a major and protracted impact to supply chains."

The Army Corps of Engineers is on the scene and will lead the effort to clear the channel.