Royal Dutch Shell has said it is working to contain an oil leak at its Gannet Alpha platform in the North Sea, but declined to specify the size of the leak.
"We can confirm we are managing an oil leak in a flow line that serves the Gannet Alpha platform. We deployed a remote-operated vehicle to check for a subsea leak after a light sheen was noticed in the area," a Shell spokesman said.
"We have stemmed the leak significantly and we are taking further measures to isolate it. The subsea well has been shut in, and the flow line is being depressurised."
Asked about the size of the leak, a Shell spokeswoman declined to say.
One of the wells at the oilfield 112 miles east of Aberdeen has been closed, but Shell did not specify whether output was reduced.
According to Argus Media, the Gannet field produced about 13,500 barrels of oil between January and April. The field is co-owned with US firm Exxon and operated by Shell.
A document available on Shell's website says the Gannet facilities have capacity to export 88,000 barrels of crude oil per day.
Shell also said it had restarted its North Sea Brent Alpha and Bravo fields on Thursday after a seven-month shutdown, while two other fields remained shut.
The company shut all four of its Brent platforms, Alpha, Bravo, Charlie and Delta, in January for repairs.
"Brent Alpha and Brent Bravo are producing gas for export via the Flags(far north liquids and gas line) to the St Fergus gas plant," Shell said.
"It is anticipated that Brent Delta will resume export in the near future and Brent Charlie will restart in early 2012."
The statement said that the work at the Brent fields was technically challenging and depended on the weather in the area.
Green party co-leader Patrick Harvie said: "It's too early to tell how serious this spill is, but it is imperative now that Shell act both urgently and efficiently.
"They must also keep the public and the authorities properly informed about progress, something BP failed to do during the Gulf of Mexico disaster last year.
"Whatever the outcome of this incident, it certainly underlines the need for the oil industry to publish proper response plans, as Greenpeace have been asking them to do. If they refuse to do so, ministers should act to make it a condition of their licences."
Before the shut-in, the four Brent fields produced about 4.5 million cubic metres a day of gas, less than 2 percent of current UK gas demand, and just 20,000 barrels per day of oil.
Brent was once Britain's largest oilfield, and still has global significance as one of the four key North Sea crude streams along with Forties, Oseberg and Ekofisk.
