Emergency workers have rushed to the scene of a dramatic bridge collapse in the United States after a cargo ship hit a pylon in Baltimore Harbour, north-east of Washington DC.

A large span of the three-kilometre-long Francis Scott Key Bridge fell into the water after a Singapore-flagged vessel crashed into the pylon about 1.30am local time, sending multiple vehicles into the water.

Authorities said it was a “developing mass casualty event” and they were trying to rescue at least seven people. A water search is under way in the Patapsco River for survivors.

“This is a dire emergency,” Baltimore Fire Department spokesman Kevin Cartwright said. “Our focus right now is trying to rescue and recover these people.”

Multiple vehicles were on the bridge when the 288-metre vessel named the Dali hit it, Cartwright said. One of the vehicles was the size of a tractor-trailer.

He said there appeared to be “some cargo or retainers hanging from the bridge”, creating unsafe and unstable conditions, and that emergency responders were operating cautiously as a result.

Fire crews told local media that as many as 20 construction workers may have been on the bridge when it collapsed.

The bridge is almost three times as long as the Sydney Harbour Bridge and forms part of a critical road link between Washington DC and New York City.