It was unclear what caused the fire at the Chevron plant in Pembroke Dock, south west Wales, on Thursday evening but one report indicated that two petrol tankers collided inside the refinery, killing both drivers.
A separate report claimed that the blast was triggered by a build-up of gas in a storage tank, near which the two refinery workers were standing.
There were fears that more may have died in the resulting blaze, with Chevron attempting to account for all of its 1,400 employees at the site.
Morriston Hospital in Swansea said one man had been brought in but there was no information available about his condition, and the hospital was not expecting any further casualties.
Coastguards reported a “blast large enough to shake the windows” while eyewitnesses described hearing a “massive bang” as a fireball erupted at the refinery around 6.15pm, followed by plumes of black smoke.
The Mid and West Wales Fire and Rescue Service said 10 engines attended the plant and an air ambulance was deployed.
A spokesman for the Welsh Ambulance Service confirmed paramedics had dealt with casualties at the scene but the heat of the flames meant it was not certain how many people were harmed.
A spokesman for Chevron said: "At 18:20 local time on June 2 2011 an incident occurred at the Pembroke Refinery.
"Emergency services were called and responded immediately and remain on the scene.
"The fire has been extinguished.
At 6:20 this evening (17:30 GMT) an incident occurred at the Pembroke refinery," Chevron confirmed in a statement.
"Emergency services were called and they remain on the scene. The fire has been extinguished.
"Appropriate action has been taken to respond to the situation," continued the statement.
"We are still in the process of accounting for all the personnel. Additional information will be provided as soon as it becomes available," the oil company added.
According to a witness, a thick plume of smoke could be seen for kilometres around.
"I was leaving the site at the end of the day and I could here explosions and see plumes of smoke," he told the BBC.
"You could see flames on the pipework from across the valley," he added.
Local fire and police services could not confirm if there had been any casualties.
