About 25,000 barrels of oil spilled into the Norwegian sector of the North Sea at the Statfjord oilfield on Wednesday and rough weather hindered the clean-up, operator StatoilHydro and energy officials said.
The coastal authority said it saw only a small chance that the oil would reach Norway's coast but predictions were uncertain. StatoilHydro said the weather was so harsh that clean-up vessels headed for the site could not scoop up the oil.
"Observations from aircraft and helicopters indicate that the oil slick is currently about eight kilometres long by one wide," Norwegian energy group StatoilHydro said in a statement.
The spill occurred while oil was being loaded from a storage unit to a tanker and has been halted, oil safety officials said.
"Four vessels are on the way out with skimming equipment but unfortunately because of the weather we cannot collect the oil right now," StatoilHydro's spokesman Kai Nielsen said. "There is too much wind, too high waves."
The company is discussing with the authorities whether to spray the oil spill with chemicals from the air to break it up into particles that would then sink, Nielsen said.
Winds are blowing towards the north, officials said.
"We see the probability as being small that this spill will reach the shores," director Tor Christian Sletner of the Norwegian Coastal Administration told Reuters.
"StatoilHydro is responsible for cleaning up. This is the 'polluter pays' principle," Sletner added.
The spill was the second largest in Norwegian oil history, the Petroleum Safety Authority (PSA) 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.
Environment Minister Erik Solheim said in a brief statement: "I see this as an unwanted and worrisome event and will follow the development closely with the Pollution Control Authority."
StatoilHydro said in a statement: "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." A cubic metre of oil is equal to 6.29 barrels.
Neither output nor exports from Statfjord, the biggest oilfield ever found off Norway though now far off its 1987-1993 peak of over 700,000 barrels per day, would be affected, the group said.
Statfjord itself now produces about 100,000 barrels per day, while around 200,000 bpd goes through it including oil from the Snorre field, and news of the spill initially pushed oil prices higher.
The spill happened in rough seas while the tanker Navion Britannica was loading oil from a storage buoy, StatoilHydro said. The vessel belongs to Vancouver-based Teekay Corp.
"We are working closely with StatoilHydro and the authorities to find out what happened," Teekay Shipping's spokesman Axel Wiedenmann said.
StatoilHydro is investigating what went wrong.
"We consider this to be very serious...that it could happen at all," Nielsen said. "We are going to do anything we can to find out what happened and to prevent it from happening again."
Winds at Statfjord are around 45 knots, and seas are around seven metres (23 feet). The field lies 200 km (124 miles) west of Bergen near the UK boundary line in the North Sea.
StatoilHydro shares, which have been hammered in past days due to a weaker production outlook, ended 1.7 percent higher at 165.40 crowns despite the spill. The Oslo bourse benchmark index was up 0.4 percent and the DJ Stoxx oil and gas index was up 1.2 percent.