Ground staff at six Norwegian airports began an indefinite strike over pay on Friday, stranding up to 20,000 passengers before Norway's biggest holiday weekend, civil aviation authority Avinor said.

The airport in Bergen, Norway's second largest city, was closed and the stoppage threatened to affect the main international airport in Oslo, Avinor said.

"There are a lot of people travelling to be with their families on Constitution Day (May 17), so this conflict is affecting thousands," Avinor spokesman Sindre Aanonsen said, adding that between 15,000 and 20,000 passengers were stranded.

"If the strike continues it may affect Oslo," he said. The capital's Gardermoen Airport is Scandinavia's second busiest hub after Copenhagen.

Other airports affected were Harstad, Narvik, Molde, Mo i Rana and Mosjoen.

"We have made our offer and it was rejected by the unions. It is out of our hands and it's really impossible to say when this will be resolved," Aanonsen said.