Royal Dutch Shell today said it was working to contain an oil leak at its Gannet Alpha Platform in the UK 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 Shell-operated 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 de-pressurised."

Asked the size of the leak, a Shell spokeswoman declined to say.

One of the wells at the Gannet oilfeild, 180km (112 miles) east of Aberdeen, has been closed. But Shell did not specify if output was reduced.

According to Argus Media, the Gannet field produced about 13,500 barrels of oil in January-April. The field is co-owned with US major Exxon and operated by Shell.

A document available from Shell's website says the Gannet facilities have capacity to export 88,000 barrels of crude oil per day.

Shell also said in a statement 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 Brent Delta will resume export in the near future and Brent Charlie restart in early 2012."

The statement said the work at the Brent fields was technically challenging and depended on the weather in the area.

Before the shut-in, the four Brent fields produced about 4.5 million cubic metres a day of gas, or less than 2 per cent of current UK gas demand, and just 20,000 barrels per day of oil.

Brent was once Britain's largest oilfield.

It still has global significance as one of the four key North Sea crude streams along with Forties, Oseberg and Ekofisk, comprising BFOE benchmark, which is used for pricing around two thirds of internationally traded crude oil.

Reuters