Seven people were missing after a fire and powerful explosion levelled a commercial fireworks facility in rural northern California, authorities said on Wednesday.

The explosions occurred at a facility in Yolo County, northwest of Sacramento, the state capital.

The blast, belching out a pall of thick black smoke, was followed by a flurry of dozens of smaller explosions flashing in the sky over the site, apparently ignited by the inventory of pyrotechnics stored at the location.

The cause of the incident was under investigation by the state fire marshal's office, Curtis Lawrence, Fire Chief for the Esparto Fire Protection District, said.

It was not immediately clear whether the missing were workers from the facility or bystanders.