A meteorologist at the Storm forecasting centre said the spill may be drifting east to southeast. That could put it on a collision course with the southwest coast of Norway.

"This could be the second largest spill in Norwegian oil history," the Petroleum Safety Authority's (PSA) spokeswoman, Inger Anda, said. The biggest was a 75,000-barrel spill from the Bravo blowout in 1977.

By comparison, the Exxon Valdez tanker spilled about 240,000 barrels of crude off Alaska in 1989.

"During loading of oil from the Statfjord A platform in the North Sea about 4,000 standard cubic metres of oil was released into the sea," Norwegian energy group StatoilHydro said in a statement.

Neither production nor exports from Statfjord, the biggest oilfield ever found off Norway though now far off its peak, would be affected, StatoilHydro said.

Statfjord currently produces about 100,000 barrels per day and news of the spill initially helped to send oil prices higher.

The spill happened in rough weather while the tanker Navion Britannica was loading oil from a storage buoy, StatoilHydro said. The ship belongs to Vancouver-based tanker group Teekay Corp.

Winds at Statfjord are for the moment around 45 knots, and seas are around seven metres (23 feet), StatoiHydro said.

The Statfjord field lies about 200 km (124 miles) offshore, west of the port of Bergen near the UK boundary line in the North Sea.

The PSA said it established an emergency response centre.

The Storm centre official said southerly near gale to gale winds were expected in the area for the next 24 hours and seas of 4-1/2 to 7 metres and the spill seemed to be drifting east to southeast, but that could not be immediately confirmed.