Energy major Equinor shut four of its Norwegian offshore oil and gas fields on Monday as its workers expanded their strike, a company spokesman told Reuters.
Two more fields operated by Neptune Energy and Wintershall Dea also face likely shutdowns on Monday because of the strike, the Norwegian Oil and Gas Association (NOG) has said.
"There is no solution in sight," an NOG spokesman said.
The escalation could cut Norway's petroleum production capacity by as much as 330,000 barrels of oil equivalent per day (boepd) or 8% of total output, said the NOG, which negotiates wage pacts on behalf of the industry.
The conflict began on Sept. 30 when a group of 43 workers organized by the Lederne union went on strike, but did not initially hit energy output.
The escalation added 126 union members to the strike, taking the total to 169 out of the 1,003 offshore workers Lederne represents.
Output from Equinor's Gudrun, Gina Krog and Kvitebjoern fields, as well as Kvitebjoern's satellite Valemon field was shut, the company said.
The Neptune Energy-operated Gjoea field and its satellite Vega field, operated by Wintershall Dea, are also likely to shut, the NOG added.
Neptune and Wintershall Dea did not immediately respond to requests for comment when contacted by Reuters.
Output from Johan Sverdrup, the North Sea's largest producing oilfield, was unaffected by the strike, Equinor said.
"Employers are still showing no willingness to meet our demands, thus triggering the escalation," Lederne trade union chief Audun Ingvartsen said in a statement.
Lederne said it was seeking better financial terms for members and wanted the offshore wage pact to also cover workers at onshore remote control rooms.
Equinor and other oil companies have been looking into ways to remotely control production at offshore fields, to cut costs.
NOG said Lederne's demand fell outside the scope of the offshore wage agreement.
(Reporting by Nerijus Adomaitis; Editing by Terje Solsvik and Clarence Fernandez)