A major fire at Shell’s biggest oil refinery — which processes 500,000 barrels of oil a day and one-third of Singapore’s oil — has forced the company to shut down most of its units.

Smoke billows from Royal Dutch Shell's Pulau Bukom offshore petroleum complex in Singapore at dusk Thursday. (TIM CHONG/REUTERS)

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