A fire and a subsequent explosion at a former air-raid shelter in the northern German city of Hamburg where plant oils were being stored have left 43 people injured.

The city's fire service said the blaze broke out early in the morning in the six-storey structure that dates back to the Second World War, news agency dpa reported.

It was not immediately clear what caused the blaze in the building, where around 100 tons of essential oils were stored and a carpentry workshop also was located.

The fire brigade evacuated more than 60 people from nearby residential buildings after being alerted to the blaze shortly before 4.30am local time.

About 2.5 hours later, an explosion occurred as firefighters tried to extinguish the blaze.

The injuries ranged from smoke inhalation to suspected broken bones. Twelve of the injured were firefighters and 19 people were taken to hospitals, the fire service said.

Authorities tried and failed to extinguish the fire by flooding the building with foam.

Later firefighters with breathing equipment were sent in to tackle the blaze.