A highway bridge over a river in the US state of Washington has partially collapsed, sending cars and people plunging into the water below.

At least three people have been rescued, but there are no reports of serious injuries or fatalities following the incident.

It's thought a high-sided truck hitting a bridge span may have caused the four lane freeway to collapse.

Pictures show a large section of the 4-lane wide bridge, between Seattle and the Canadian city of Vancouver, in the water, with traffic and crowds of people visible on the end nearest land.

The collapse happened in a rural part of the state.

Over-height load

"We are looking at a potential over-height load," Travis Phelps, a spokesman for the Washington State Department of Transportation, told Kiro 7 television.

Witness Dale Ogden said he saw the truck strike the bridge.

"I was less than 50 feet away from the truck when it hit it ... I had just passed it in the fast lane southbound and it had an oversized load," he told KING 5 television.

"It was approximately 12f wide and over 14ft tall ... I saw the truck strike the right corner of the bridge. It almost tipped the truck over but it came back down.

"It tipped it up to about a 30-degree angle to the left and it came back down on its wheels and almost instantaneously behind that I saw girders falling in my rearview mirror."

The bridge is not considered structurally deficient but is listed as being "functionally obsolete" - meaning its design is outdated, according to the Federal Highway Administration. Built in 1955, the bridge has a sufficiency rating of 57.4 out of 100 - well below the state average of 80, data shows.