The union said some 1,115 workers were planning to go on strike, in addition to 1,422 who went on strike this Monday after negotiations with employers broke down last weekend.

If the escalation goes ahead as planned some 655 union members plan to stop work at Norsk Hydro's primary aluminium plant in Sundal, and 444 members at Alcoa's aluminium plant in Mosjoeen.

"We are putting increased pressure on employers to start the negotiation process again," Industri Union's leader Frode Alfheim said.

The strike, which started on Monday, is initially targeting several electrochemical plants, including Boliden's zinc smelter in Odda and Glencore's nickel refinery in Kristiansand.

The union has 7,600 members across the electrochemical industry and represents them in their wage negotiations.

(Reporting by Nerijus Adomaitis; editing by David Evans)