(Adds reaction by Norwegian Shipowners' Association)
OSLO, July 16 (Reuters) - A Norwegian union for workers on offshore oil and gas drilling rigs stepped up a six-day strike on Monday that has hit oil output after employers did not respond to demands for higher wages and pension benefits.
The union is adding 900 workers to the strike, under a plan announced last week, after failing to win concessions before a midnight (2200 GMT) deadline since almost 700 workers on the rigs went on strike on Tuesday.
"The escalation takes place from midnight as planned," Safe union spokesman Roy Aleksandersen said just before the deadline. The workers on the offshore oil and gas rigs went on strike after rejecting a proposed wage and pension deal.
Employers, represented by the Norwegian Shipowners' Association, said on Monday the union "lacked understanding of the economic reality of the Norwegian continental shelf."
"It is ... utterly incomprehensible that Safe is willing to put at risk jobs, the shelf's attractiveness and competitiveness, and Norway's reputation as a safe and stable supplier of oil and gas resources," lead negotiator Jacob Korsgaard said in a statement.
On Saturday, Korsgaard told Reuters the Shipowners' Association planned no compromise talks. "Obviously a conflict like this cannot go on forever. All counter-measures are considered," he said, without giving details.
The expanded strike is not expected to have any immediate extra impact on oil or gas production beyond the closure last week of Shell's Knarr field, which produces 23,900 barrels of oil equivalent per day.
Both the Safe union and employers in the Norwegian Shipowners' Association said late on Sunday they had no contacts or new offers during the weekend.
The employees joining the action work on exploration and production drilling rigs owned by Saipem, Transocean , Songa Offshore, Odfjell Drilling, Archer and COSL, among others. (Editing by Alister Doyle and Louise Heavens)