A major bridge in the US city of Baltimore collapsed on Tuesday after a ship collided with it, crashing into water below and seemingly bringing vehicles with it.

Dramatic CCTV footage shows what appears to be a container ship hitting a footing of the Francis Scott Key bridge, sending it plunging into the Palapsco river, followed by the greater part of the massive structure.

Lights from what appear to be vehicles can be seen on the road surface as the bridge warps and crashes in sections, with the third tranche cantilevering upwards before it, too, tumbles into the water.

"We have reason to believe that there were vehicles and possibly a tractor-trailer that went into the water,"  Kevin Cartwright of Baltimore Fire Department told the Baltimore Sun, adding "the entire bridge" had collapsed.

A Baltimore Police Department spokesman told NBC News that people were possibly in the river.

"I can confirm at 1:35 a.m., Baltimore City police were notified of a partial bridge collapse, with workers possibly in the water, at the Francis Scott Key Bridge," Detective Niki Fennoy said.

The 1.6-mile (2.6-kilometer), four-lane bridge spans the Patapsco River southwest of municipal Baltimore. 

It opened in 1977 and carries more than 11 million vehicles a year.

It is a major part of the road network around Baltimore, an industrial city on the US East Coast next to the capital Washington DC.

The Maryland Transportation Authority told drivers to avoid the bridge, part of the I-695 interstate highway, which it called an "active scene".

Ship monitoring website MarineTraffic showed a Singapore-flagged container ship called the Dali stopped under the bridge early Tuesday. 

Baltimore Mayor Brandon Scoot and Baltimore County Executive Johnny Olszewski Jr both said they were aware of the incident and that rescue efforts were underway. 

"Please pray for those impacted," Olszewski posted on X.

Scott said he was "en route to the incident."