Norwegian trade union Industri Energi will expand the ongoing strike among Norwegian oil service workers by 23 people, adding to the 335 that went on strike on Sept. 21, a union spokesman told Reuters on Sept. 23.

The strike affects operations of subcontractors to the oil industry and could have consequences for the output of oil and gas in case of a protracted strike, the union and employers have both said.

"The strike has already halted a large number of drilling operations," said an official at the Industri Energi trade union, Ommund Stokka. It was not affecting oil and gas output at this stage, he added.

Union officials would meet next week to review the state of the strike and what to do next, he said.

Mediation between Industri Energi and the Norwegian Oil and Gas Association broke down earlier this week, triggering the initial strike at the Norwegian units of Schlumberger, Halliburton, Baker Hughes, Oceaneering and Oceaneering Asset Integrity.

The negotiations were on behalf of about 6,500 union members at about 30 companies.

Norway is western Europe's top oil and gas producer, and state-controlled Statoil ASA (NYSE: STO) is its largest firm.