A highway bridge over a US river has collapsed, dumping vehicles and people into the water, police say.

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A portion of the Interstate 5 bridge is submerged. Picture: AP

The major highway bridge linking Seattle with Canada and the rest of the Pacific Northwest region has collapsed, dumping at least a handful of vehicles and people into a river.

Washington State Patrol said no one died, but three people were taken to hospitals.

The four-lane Interstate 5 bridge - more than half a century old - collapsed on Thursday night about 7pm local time, halfway between Seattle and Vancouver, British Columbia, Trooper Mark Francis said.

Francis said he did not know what caused the collapse, which came at the start of one of the country’s busiest holiday weekends of the year.

Kari Ranten, a spokeswoman for Skagit Valley Hospital, said two people who were injured in the collapse were en route to the facility. She said another person was being taken to a different area hospital.

Xavier Grospe, who lives near the river, said he could see three partially submerged cars, and the apparent drivers sitting either on top of the vehicles or on the edge of open windows.

“It doesn’t look like anybody’s in danger right now,” Grospe said.

A man sits atop a car that fell into the Skagit River after the collapse of the Interstate 5 bridge. Picture: AP/Francisco Rodriguez

Helicopter footage aired by KOMO-TV in Seattle showed one rescue boat leaving the scene with a person strapped into a stretcher. A damaged red car and a damaged pickup truck were visible in the water, which appeared so shallow it barely reached the top of the car’s bonnet.

A man told the local Skagit Valley Herald newspaper he felt a vibration and looked in his rear view mirror to see that the part of bridge he had just crossed was no longer behind him.

“I thought something was wrong with my car at first,” he said.

The water in the Skagit River is believed to be about 4C at this time of year.

The bridge is not considered structurally deficient but is listed as being “functionally obsolete” - meaning its design is outdated, according to a database compiled by the Federal Highway Administration.

The American Society of Civil Engineers’ 2013 infrastructure report card said more than a quarter of Washington state’s 7840 bridges were structurally deficient or functionally obsolete.

Vehicles and people were dumped into the water. Picture: AP