OSLO - Some 573 members of Norway's Industri Energi oil and gas union plan to go on strike from June 12 onwards if state-brokered mediation with employers fail, but output would initially remain unaffected by a conflict, the trade union said on Friday.

The union members plan strike action at nine offshore installations, including the Njord A, Valhall and several Oseberg platforms, the union said in a statement.

"The first instance of a potential strike would only involve a limited number of members, but we can escalate if necessary," Industri Energi's chief negotiator Lill-Heidi Bakkerud said.

Industri Energi is Norway's biggest oil and gas union, negotiating on behalf of some 4,300 members.

Two smaller labour unions that will be part of the June 10-11 wage mediation have yet to outline their response to any breakdown in the talks.

(Reporting by Nerijus Adomaitis and Nora Buli, editing by Terje Solsvik)