At least seven people died when dozens of vehicles piled into each other on the M5, triggering a fireball that hampered initial rescue attempts, police said on Saturday.

More than 50 people were injured in the Friday evening incident which was described by police as horrific. They warned the death toll could rise as extensive searches were made of vehicles and the carriageway.

This is a hugely complex operation because of the scale of vehicles and people involved, Assistant Chief Constable Anthony Bangham told reporters. For a motorway incident, this is just about as big as it gets.

About 34 vehicles, including lorries and cars, smashed into each other on the northbound carriageway of the M5 motorway near Taunton, Somerset.

Fires broke out instantaneously, in what was described by the Devon and Somerset Fire and Rescue Service as the worst traffic collision rescue workers could remember.

The seven people all died at the scene of the crash, police said. Bangham praised the bravery of witnesses who had tried to rescue those trapped, before they were beaten back by the intensity of the flames.

There was some real bravery from people trying to help others, he said. The intensity of the fire, it was a fireball on the carriageway, made it incredibly difficult for people to approach. But people did their very best.

The cause of the accident remains unknown, though some witnesses said the conditions had been foggy and wet.

Many of the vehicles were burnt beyond recognition.

It is a real mash of many, many vehicles, Bangham said.

It was obviously a very considerable impact. (It is) quite unusual to have such a fireball, and certainly the intensity.

The motorway, the main gateway to England's south west, is likely to remain closed in both directions until at least Sunday, he said.

(Reporting by Avril Ormsby; Editing by Sophie Hares)