"This could be the second largest spill in Norwegian oil history," the Petroleum Safety Authority's (PSA) spokeswoman Inger Anda said.

She said the spill was estimated at 3,840 cubic meters of oil, which corresponds to 24,154 barrels of oil. The biggest oil spill ever off Norway occurred in the Bravo blowout in 1977 when around 12,000 cubic meters of oil were spilled, Anda said.

StatoilHydro information director Ola Morten Aanestad confirmed that the company had had a spill at its Statfjord Alpha platform, about 200 kilometers west of Bergen, near the border of the British continental shelf.

StatoilHydro said the spill was about 4,000 cubic meters, and occurred in connection with the tank ship "Navion Britannica" loading aboard oil from a loading buoy.

StatoilHydro spokesman Vegar Stokset said the cause of the spill was not immediately known but the loading had immediately been stopped.

"It is a significant amount and we are taking it seriously," said Stokset, adding that production from the field was not affected because tanker loading is a separate operation.

The Norwegian Rescue Coordination Center South had been notified and planes and helicopters from the Norwegian Pollution Control Authority (SFT) were deployed to get an overview of the extent of the spill.

Weather in the area is poor, with 45 knot (51.8 pmh) winds and choppy seas.

Meteorologist Hilde Holdhus at the Storm Weather Center said the conditions were good for seabirds, as they would prevent the oil being blown inland. The winds will make it more difficult for aircraft to get an overview of the situation.