Norwegian oil and gas workers have walked off the job amid concerns about Europe's gas supplies.
"The strike has started," said Audun Ingvartsen, chairman of the Lederne union, on Monday evening.
Oil and gas production will be cut by 89,000 barrels of oil equivalent per day (boepd) as a result of Tuesday's work stoppage, of which gas production will account for 27,500 boepd, Norwegian energy group Equinor said.
According to a calculation by the Reuters news agency, a planned gradual reduction in production by Saturday could result in almost a quarter of Norwegian gas production and around 15 percent of oil production being shut down.
The decision to cut production ultimately rests with the operator Equinor.
The group could not initially be reached for comment.
The Norwegian government said it was following the conflict "closely" and could end the strike in "extraordinary circumstances".
For the EU countries, the work stoppage comes at an inopportune time, as they are dependent on more gas from Norway due to a lack of Russian supplies.
Most recently, the trading gas price had risen sharply and doubled within a month to more than 160 euros per megawatt hour (TTF).