Anglo-Dutch energy giant Shell said Thursday that a major Singapore refinery was on fire and that firefighters have contained the blaze but were still working to completely extinguish it.

"We believe it was an accident. A full investigation will be conducted once the fire is put out," the company said in a statement.

The fire broke out Wednesday at the Shell refinery in Pulau Bukom, an islet five kilometers (three miles) off Singapore, and prompted the evacuation of non-essential staff.

About 100 firefighters from the Singapore Civil Defence Force (SCDF) and 250 "essential" Shell personnel worked overnight to contain the blaze.

The plant, Shell's biggest in the world in terms of crude distillation capacity, produces fuels, lubricants and specialty chemicals, mostly for export.

"The fire at Pulau Bukom Manufacturing Site... has been contained. We are working closely with the Singapore Civil Defence Force to put out the fire," Shell said.

"All staff are accounted for and non-essential staff have been evacuated to safety."

Shell said one company firefighter "sustained a superficial injury, and five other firefighters had heat exhaustion and pulled muscle."

Singapore's civil defence force said in a separate statement that the fire involved petroleum products from pipes in a "tank farm" at the facility.

The blaze has been contained within an area measuring 150 metres by 50 metres (495 feet by 165 feet), the SCDF said.

"The situation is now under control. However, firefighting operations are still underway at the affected area," it added.

SCDF said 21 water jets were being used for cooling operations to prevent nearby storage tanks from being exposed to the heat.