Part of a four-lane bridge over a river in the US state of Washington has collapsed, sending vehicles and drivers tumbling into the water.

Two of the three people rescued from the river were hospitalised with hypothermia.

There were apparently no fatalities.

Authorities are awaiting confirmation on the cause of the collapse.

Local media reported it might have been caused by a truck striking the structure.

The bridge is on the Interstate 5 where it crosses the Skagit River between the towns of Mount Vernon and Burlington, 90km north of Seattle.

The road is the main corridor for car traffic between Seattle and Vancouver, Canada.

The bridge was built in 1955, according to the website for the National Bridge Inventory Database.

In August 2007, a bridge fell in Minnesota resulting in the deaths of 13 people, which raised concerns about faulty infrastructure in the US.

That incident saw about 305m of the I-35W bridge in Minneapolis collapse into the Mississippi River during evening rush hour.

The US National Transportation Safety Board subsequently found more than a dozen steel support plates suspected of causing the disaster were deficient in size, and a routine inspection would not have uncovered the problem.