OSLO, NORWAY - A FIVE-DAY-OLD strike at six airports in Norway was extended to 12 airports on Tuesday that include those providing helicopter services to the offshore oil sector, airport operator Avinor said.

The airport in Stavanger, which serves North Sea oil workers, and those in five other towns cancelled all flights, Avinor said.

The strike - launched on Friday after wage negotiations broke down between Avinor and the main trade union confederation LO - left around 31,000 passengers stranded on Tuesday.

It was also complicating rotations on oil platforms out in the North Sea, where staff usually work two-week shifts. There was no immediate impact on oil production.

The Scandinavian country is the world's fifth-biggest oil exporter and third-biggest gas exporter.

The dispute had until Tuesday affected six airports, including the one in Norway's second biggest city Bergen, another big transport hub for the offshore oil industry.

Air traffic at Oslo international airport was only marginally affected.

The government has so far ruled out an intervention, preferring instead for the two sides to find a mutual agreement.

In Norway, authorities can force strikers back to work if a work stoppage poses a threat to health or security or is damaging to society - an option often used to maintain oil and gas production. -- AFP