Four people have been killed and one seriously injured in an explosion at an oil refinery in Pembroke, southwest Wales.
Police said a fire had broken out at the Chevron-owned refinery after an explosion in a storage tank during maintenance, and an adjacent storage tank was also damaged.
"I was in the back garden and I just heard a large explosion, looked around and I could see a fireball rising up into the sky," local resident Phil Horne told the BBC, adding that smoke billowed out for between 30 and 45 minutes.
The fire service said it sent 10 emergency vehicles to the scene and the ambulance service said it responded with three ambulances and an air ambulance.
US-based Chevron said the 220,000 barrel per day refinery remained fully operational following the explosion, and would take "every step possible to determine the series of events that led to this tragic incident".
Chevron put the refinery up for sale last year to lower its exposure to the refining business, echoing similar moves by rivals such as Royal Dutch Shell and BP.
Falling oil demand in Europe has made it the least attractive location for refinery investment.
U.S. refiner Valero Energy agreed in March to buy the refinery and other assets in a $730 million deal expected to close in the third quarter.
Valero was not available for comment on whether it would go ahead with the purchase.
An investigation is due to be launched by Britain's Health & Safety Executive (HSE) regulator and the Environment Agency.
Concerns over refining safety have increased in the oil industry since an explosion at a BP refinery in Texas City in 2005, after which BP said safety shortcomings were common across the sector.
According to an HSE's enforcement database, the Pembroke plant had received one enforcement notice in recent years.
This criticised the plant for "failure to demonstrate the company had identified safety instrumented systems", though the HSE noted the enforcement notice had been complied with.
By comparison, four enforcement notices were issued to the Coryton refinery in southeast England, which was owned by BP before being sold to Swiss-based Petroplus Holdings, and three notices were given to Exxon Mobil's Fawley refinery in southern England.