* Norway union, employers have no contacts over weekend
* Some 900 workers join rig strike from Monday
* Strike to have no immediate extra impact on oil, gas output
OSLO, July 15 (Reuters) - A Norwegian union for workers on offshore oil and gas drilling rigs stepped up a six-day strike on Monday that has slightly 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 expanded strike will not 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 websites of both sides, which they said would notify workers of any breakthroughs, had no updates as the deadline passed.
"The escalation takes place from midnight as planned," Safe union spokesman Roy Aleksandersen said just before deadline. The workers on the offshore oil and gas rigs went on strike after rejecting a proposed wage and pension deal.
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. (Reporting by Lefteris Karagiannopoulos, Editing by Alister Doyle and Gareth Jones)