Footage shows a large section of the 1.6 mile-long Francis Scott Key Bridge falling into the water following the collision at around 1.30am local time.

Baltimore's fire service said emergency teams are searching for at least seven people believed to have fallen into the water.

Baltimore bridge latest: Workers 'possibly in the water'

Chief Kevin Cartwright, director of communications for the Baltimore City Fire Department, said the main focus right now is "trying to rescue and recover these people".

He said it's too early to know how many people were affected but called the collapse a "developing mass casualty event".

Mr Cartwright said it appears there are "some cargo or retainers hanging from the bridge," creating unsafe and unstable conditions, and that emergency service workers are operating cautiously as a result.

"This is a dire emergency," he said.

Vehicles can be seen in the footage falling from the bridge.

A spokesperson for Baltimore Police Department has told Sky News' US partner network, NBC News, that it had been notified of the incident at the Francis Scott Key Bridge

"I can confirm at 1.35am, 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 in a statement.

Baltimore's mayor, Brandon M Scott, said emergency personnel were on the scene and rescue efforts were underway, while Johnny Olszewski, Baltimore county executive, said he remains in contact with the chief of the fire department and the director of emergency operations.

"Please pray for those impacted," Mr Olszewski said.

Built in 1977, the bridge is referred to locally as the Key Bridge. It is one of the longest continuous truss bridges in the world, according to the National Steel Bridge Alliance.

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