Workers at a scrap shop in Bangkok, Thailand, accidentally detonated a large bomb believed to have been dropped during the Second World War, killing at least seven people and injuring 19 others, police said.

Police bomb squad chief Kamthorn Auicharoen said construction workers found the bomb buried at a building site and sold it to the scrap shop in northern Bangkok. He said workers at the shop were using a gas-powered thermal cutter to take the bomb apart on Wednesday when it exploded.

The massive blast ripped apart the scrap shop in Bangkok's Lad Plakao neighbourhood and damaged nearby houses.

"It's likely this is a bomb dropped from a plane during World War II," Kamthorn said.

Construction workers on another project found a similar bomb in a different area of Bangkok just this past Saturday, but it was safely handed over to police explosive experts.

Bangkok was bombed by U.S. and British planes during the Second World War, when it was occupied by Japanese forces and served as a hub for their campaign in Southeast Asian. Buried bombs from the era continue to be found from time to time in other major Asian cities, including Hong Kong and Singapore.