OSLO, July 7 (Reuters) -
** Labour union Industri Energi on Thursday said a largenumber of workers at Norwegian oil service firms will go onstrike later this year unless a wage deal is found in upcomingmediation
** Voluntary wage talks for 6,500 union members at around 60companies broke down on Wednesday
** "A strike may hit output," Industri Energi union leaderLeif Sande told Reuters. Still, a strike will most likely bemonths away
** Workers are not allowed to strike unless mediation by astate-appointed mediator has been attempted. Industri Energi didnot specify how many of its members would initially go onstrike, but said the numbers would be extensive
** A smaller trade union, Safe, struck a deal for itsmembers late on Wednesday, but Industri Energi said it woulddemand larger wage increases than those obtained in the Safedeal
** Industri Energi also said it believed the deal signed bySafe and the Norwegian Oil and Gas Association was illegal, andadded it would take the matter to Norway's Labour Court
** Industri Energi said mediation would only take placeafter the legal matter has been resolved, and a potential strikecould consequently be months away (Reporting by Stine Jacobsen, editing by Terje Solsvik)