Bridge in US city of Baltimore collapses after ship hits it, sending ...

A major bridge in the US city of Baltimore has collapsed after a large ship collided with it, sending multiple vehicles into the water.

In the early hours of Tuesday morning local time a large vessel crashed into Francis Scott Key Bridge, catching on fire and causing multiple vehicles to fall into the Patapsco River below, according to a live stream posted on YouTube.

The Baltimore City Fire Department described the collapse as a mass-casualty incident, and said workers were searching for multiple people in the river.

There were no immediate reports of casualties.

It is not known how many people were on the bridge at the time of the collapse.(Supplied: Harford County MD Fire & EMS)

Mayor Brandon M Scott and Baltimore County executive Johnny Olszewski Jr posted that emergency personnel were responding and rescue efforts were underway.

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

Emergency responders were searching for at least seven people believed to be in the water, Kevin Cartwright, director of communications for the Baltimore Fire Department, told the Associated Press at about 3am local time.

He said agencies received 911 calls at about 1:30am, reporting a vessel travelling outbound from Baltimore that had struck a column on the bridge, causing it to collapse.

Multiple vehicles were on the bridge at the time, including one the size of a large truck.

"Our focus right now is trying to rescue and recover these people," Mr Cartwright 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)

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 responders are operating cautiously as a result.

Data from marine tracking websites show the vessel involved is the Singaporean-flagged container ship Dali, which had left Baltimore's Point Breeze before the collision occurred.

Synergy Marine Group, which owns the Dali, confirmed the ship had collided with a pillar of the bridge.

"Whilst the exact cause of the incident is yet to be determined, the Dali has now mobilised its Qualified Individual Incident response service," it said, adding that all crew members, including the two pilots who were aboard, have been accounted for and there are no reports of any injuries.

"This is a dire emergency," he said.

The Francis Scott Key Bridge opened in 1977.