Part of a four-lane freeway bridge over a river in a rural area north of Seattle has collapsed, sending vehicles and people into the water below, authorities say.

The collapse on Thursday on Interstate 5 over the Skagit River happened at about 7pm local time between the towns of Mount Vernon and Burlington, 90km north of Seattle in Washington state, Mark Francis, a patrol spokesman trooper, said.

At least three people were pulled out but there were no reports of serious injuries or fatalities in the incident, which may have been caused by a high-sided lorry hitting a bridge span, a spokesman said.

Pictures showed a large mangled section of the bridge collapsed into the water, with traffic and crowds of people visible on the end nearest land.

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

The American Society of Civil Engineers' 2013 infrastructure report card gave the country a dismal grade of D-plus overall.

"We are looking at a potential overheight load," Travis Phelps, a spokesman for the state's transportation department, told Kiro 7 television.

Interstate 5 and the bridge over the Skagit River are the main corridor for car traffic between Seattle and Vancouver, Canada.

Residents cautioned

The Skagit County Sheriff's Office, in a statement on its Facebook page, asked people to avoid the area to make room for emergency responders.

"Please try to stay off land and cellphones as the volume of calls has overloaded the system and first responders are not able to make needed calls due to line overload," the statement advised residents.

The collapse comes nearly six years after another bridge fell in Minnesota and raised concerns about faulty infrastructure in the US.

In August 2007, about 305 metres of the I-35W bridge in Minneapolis collapsed into the Mississippi River during the evening rush hour. Vehicles plummeted into the water, killing 13 people and injuring another 145.

In a subsequent investigation of the fatal collapse, the US National Transportation Safety Board found that more than a dozen steel support plates - suspected of causing the disaster - were deficient in size.

It also said that a routine inspection would not have uncovered the problem.

Thursday's bridge collapse will raise questions about the nation's infrastructure, which has been a popular issue with President Barack Obama, who earlier this year sounded a warning on "raggedy" roads and sought to focus more money on rebuilding to improve the economy.

Source: Agencies