The Subway restaurant logo is seen on a napkin in this illustration photo August 30, 2017. [Source: Reuters]

At least six people were killed and several injured on Saturday when a fire broke out following an explosion at an oxygen plant in southeastern Bangladesh, officials said.

The death toll could rise as a rescue operation was ongoing at the plant at Sitakunda, 40 km (25 miles) from the southeastern port city of Chittagong, they added.

It was not immediately clear what caused the explosion, a fire service official said.

A huge bang was heard which shook an area with a radius of up to two kilometers, police official Nayhanul Bari said, quoting witnesses.

A massive fire at a container depot in the area killed 50 people and injured nearly 200 in June last year.