A Statoil spokesman said production was cut by around 38,000 barrels per day after oil union NOPEF stepped up its strike over the weekend to include about 415 workers, mostly employed in well operations. The dispute is over wages and working conditions and affects the Statfjord and Gullfaks fields and the Norwegian Sea Heidrun field.

"In total, the consequence of the strike is that production is reduced by up to 38,000 barrels a day, of which Statoil has about 50 percent," Kristofer Hetland was reported to have said. "At old oilfields we need well maintenance to maintain production," he said. Hetland said Statoil was aiming to compensate for the lost production after the strike, adding that the financial consequences of the strike would be "relatively limited".

Norsk Hydro, the second biggest producer on Norway's continental shelf, said it had halted drilling at three out of four rigs at Troll and delayed planned production drilling at Oseberg South, both in the North Sea.

"Our production has so far not been directly affected, but drilling has been halted and postponed," Norsk Hydro spokeswoman Hege Norheim said. "But if we see a long-lasting strike, it could have direct impact on production."

Oil service workers, saying they might step up the strike even further, demand similar wages and working conditions as other offshore staff. "We will continue the strike, we might even step it up further today or tomorrow," NOPEF spokesman Reidar Sivertsen said. "We feel that the strike is effective."