An explosion at an oil refinery in the heart of Bangkok which sparked a massive fire early Wednesday has triggered calls for the relocation of the 120,000 barrels-a-day facility.

The government closed the Bangchak Petroleum refinery for at least 30 days pending an investigation into the blaze, which sent flames and a thick column of smoke into the sky above the city of 13 million people.

There were no reported injuries in the blast and subsequent fire, but the incident prompted Bangkok's governor to speak of his "dream" of relocating the refinery to the city outskirts from its current site in an industrial area situated in a densely populated suburb.

The refinery was built 50 years ago in what was then the periphery of the city, but as Bangkok's population expanded it has become encircled by several neighbourhoods, stoking fears over the potential safety risk it poses to the public.

"In the past, the Bangchak refinery was outside of town but now there are millions people around here," Governor Sukhumbhand Paribatra said.

"My dream is to receive cooperation from Bangchak to move out from here so that we could turn this place into a public park... but this is the dream for future, not today."

His comments where echoed by a local environmental organisation which alleged a similar incident occurred last year and called on the government to make plans to relocate the massive facility.

"I have written to government today and asked them to map out (a) plan to move Bangchak refinery out of Bangkok because it has increased life risks for nearby residents," Srisuwan Janya, of the Stop Global Warming Association said, threatening legal action to force the government to act.

The oil refinery employs around 600 people and processes tens of thousands of barrels of fuel each day.

Early investigations by Bangchak Petroleum -- Thailand's second biggest refiner -- suggested the blast was caused by an oil or gas leak.

"We assume oil or gas leaked out and somehow it caught fire," Bangchak president Anusorn Sangnimnuan told reporters, adding that the blaze was under control by mid-morning.

"I don't think the cause was human error, it was caused by equipment. There was black smoke but it won't affect the environment or surroundings."

Emergency services said there had been no casualties, but as a safety measure Bangkok authorities distributed masks to thousands of nearby residents to prevent smoke inhalation.

Responding to the blast Industry Minister Pongsawat Sawadiwat said the refinery will be shut for at least a month, but conceded it would be difficult to move the refinery permanently as it had been established before people settled in the area.

In May an explosion at a chemical factory in eastern Thailand killed 12 people, injured more than 100 and spewed plumes of toxic smoke into the air.