At least six people have died after a train derailed off a bridge onto a busy road near Seattle, a US official has said.

The Amtrak train came off the rails over a busy interstate road in Washington state on the west coast at around 8am local time (1pm GMT) on Monday, authorities said.

Ed Troyer, a spokesman for the Pierce County Sheriff, has earlier confirmed "multiple fatalities" amongst those on the train without giving any firm numbers.

The force said there were "multiple motorists injured". At least 70 people have been taken to hospital.

Amtrak said there were around 78 passengers and five crew members on board the 501 train service, the first of a new high-speed service linking West Coast cities.

Police said it was not yet clear what had caused the crash.

One US official said an obstruction on the track was seen as a possible cause.

There have also been suggestions that speed may have been a factor.

Local media reports said many ambulances has been seen departing from the scene with their emergency sirens on.

Some walking wounded were also given treatment at medical tents set up by the side of the road.

President Donald Trump said his "thoughts and prayers" were with everyone involved in the accident and the White House was monitoring the situation.

All southbound lanes of Interstate 5 were closed south of Joint Base Lewis-McChord, and motorists are being warned to avoid the area.

Amtrak trains running south from Seattle have also been suspended as emergency services deal with the accident.

The accident happened on the first day of a the new Amtrak Cascades' high-speed route between Seattle and Portland.

It saw trains moved onto a new inland route which runs in a straighter line than the previous track, meaning that trains could travel faster as they do not have to slow for bends as often.

The train was going at a speed of 81.1mph moments before the derailment, according to transitdocs.com, a website that maps Amtrak train locations and speeds using data from the railroad's train tracker app.

The maximum speed along the stretch of track, known as Point Defiance Bypass, is 79mph, according to information about the project posted online by the Washington State Department of Transportation.