Madrid - At least five people have been confirmed dead after a large blast on Monday at a fireworks factory in north-eastern Spain, authorities said.

Six other employees of the plant near the city of Zaragoza were hospitalised, some with serious injuries, said Gustavo Alcalde, prefect of the Aragon regional government.

The cause of the accident at the more than 150-year-old Pirotecnia Zaragozana factory was not immediately clear.

The explosion, which could be heard 10km away in Zaragoza, sent people running scared from their homes, the fire brigade said.

Firefighters were unable to quickly enter the site due to several smaller explosions.

Pirotecnia Zaragozana employs 40 people, according to the company website.