"The strike is having quite a big impact.... Its impact on production is 10 percent of oil production and seven percent of gas production," Eli Ane Nedreskår, a spokeswoman for the the Norwegian Oil Industry Association (OLF) said.
"It costs 200 million kroner (33.4 million dollars) a day," she said.
The stoppage of oil and gas production at Statoil's Oseberg and Heidrun fields in the North Sea has led to "a daily loss of some 240,000 barrels of oil and 11.9 million standard cubic metres of gas," Nedreskår said.
The strike began after talks broke down over employers' refusal to change a decision to cut a pension add-on for employees who retire at 62, three years ahead of the general age for oil workers and five years ahead of Norway's official retirement age, the SAFE and Industri Energi unions said.
The two sides were in contact over the weekend but no progress was made in the talks, Nedreskår said, adding that she hoped "the strike would be as short as possible."
The strike involves employees of Norwegian oil group Statoil, British oil giant BP's Norwegian division and ESS Support Services, which is owned by the British Compass Group.
Statoil is the only company affected by production stoppages.
The previous oil workers' strike in Norway took place in 2004 and lasted one week.