In less than half-an-hour, a cargo ship had left its port, suffered a power failure and destroyed Baltimore's Key Bridge. It was all captured on video.

It was the middle of the night in Baltimore, Maryland, in the north east of the United States.

A cargo ship flying the Singaporean flag left the docks at 1:04am (4:04pm AEDT) headed to Colombo, Sri Lanka.

But within 20 minutes, DALI had lost its power and all control.

As it drifted towards the Francis Scott Key Bridge, a mayday went out, prompting officials to stop traffic and save lives.

"Hold all traffic on the Key Bridge," officials on a dispatch said. "There's a ship approaching that's just lost their steering."

All lights were out on the ship and black smoke was coming out its top.

Vehicles were stopped from using the bridge but a work crew repairing potholes was not moved in time.

At 1:28:48am local time, it made impact, causing the bridge's roadway to fall down like Jenga blocks and sending vehicles and workers into the icy river water.

It took just 30 seconds for all the pieces to come to a stop.

Baltimore City Fire Department dispatched rescue teams for the Patapsco River 10 minutes later, with reports of multiple people in the water.

The first fire unit arrived on the scene at 1:50am.

The entire ordeal took about 25 minutes, from the moment the ship left Point Breeze until the last part of the bridge hit the icy water.

Rescuers pulled two workers from the water alive on Tuesday, and one of them was hospitalised but six others are presumed to have perished.

Officials said diminished visibility and increasingly treacherous currents in the wreckage-strewn channel made search efforts risky.

All 22 crew members on the ship were accounted for.

The 300-metre ship is still floating on the water with all its containers on board.

Traffic was suspended at the port after the collision, disrupting shipping in the region.

Loss of the bridge also snarled roadways across Baltimore, forcing motorists onto two other congested harbour crossings and raising the spectre of nightmarish daily commutes and regional traffic detours for months or even years to come.

The Francis Scott Key Bridge, which opened in 1977, was one of three ways to cross the Baltimore Harbour.

It was 2.5 kilometres long with the steel structure measuring four lanes wide and sitting 56 meters above the river.

The bridge's main span of 366 metres was the third longest span of any continuous truss in the world.

It's a title it lost within minutes of the collapse as the entry was quickly removed from Wikipedia.

Reporting and production: Basel Hindeleh

Additional reporting by Reuters

Video from StreamTime Live

Satellite imagery from Planet Labs