Fears over job losses at the Chevron refinery in Pembrokeshire have been raised after the oil giant said it was to "exit from certain markets".
The firm, the fifth-largest refiner in the United States, has told staff it is planning a restructuring that would involve sweeping cuts globally.
It said it had not decided whether to close any of its refineries.
The Pembroke site has 1,400 permanent and contract staff and processes 220,000 barrels of crude oil a day.
Chevron said further details on its plans were expected to be released in March.
A spokesman for the company said it is planning to restructure some of its refinery operations, cut some jobs and exit some markets.
In America, some refiners have shut down operations as the rising cost for crude oil cut profits faster than the income from the products they refine such as heating oil, diesel fuel and jet fuel.
Chevron employees have been told about the plans to restructure the firm.
"The global recession has reduced demand for refined products while global refining capacity continues to surge," explained Chevron spokesman Lloyd Avram.
"To improve profitability and long term competitiveness, Chevron has created a new downstream organisation.
"This organisation is less complex, more responsive to market opportunity and smaller, requiring fewer positions and people.
"No further decisions regarding assets or markets have been made."
He added that no decision about how many jobs would be needed after the restructure.
He said Chevron has also not decided whether to close any of its refineries.
The Chevron refinery in Pembroke, which first opened in 1964, employs 1,200, processes 220,000 barrels of crude oil a day.
The refinery is the fourth largest in the UK, with just under 12% of the UK's overall refining capacity.
In 2005, the Pembroke refinery had an £84m upgrade.
The Welsh Assembly Government's Deputy First Minister, Ieuan Wyn Jones, said: "I understand that the news that Chevron plans a global review may create uncertainty and worry, not only for the workforce at Pembrokeshire, but also their families and the wider community.
"However, no decision about any of Chevron's plants has been taken.
"The Pembrokeshire plant makes a major contribution to UK refining capacity which is still required even in these difficult times."
He said he would be visiting the plant soon and would be contacting Chevron in California, "offering any help we can to ensure the plant has a strong future".
A senior member of Pembrokeshire council said the plant "is a very important part of the Pembrokeshire economy and we hope will continue to be so".
Councillor John Allen-Mirehouse, the cabinet member for Regeneration and Economic Development, said: "Over many years, the company - and its predecessor Texaco - has made massive contributions in this part of the world not only in terms of employment but also through its active involvement in the community.
"Pembrokeshire County Council has always worked closely with the management at the refinery and will continue to do so."
Stephen Crabb, the MP for Preseli Pembrokeshire: "As the largest private sector employer in Pembrokeshire, the refinery is of critical importance to the local economy. Its loss would be a catastrophic blow to the area."