Adds third labour union, employers' organisation and current wage level paragraphs 1-3, 6, onshore workers agreement paragraph 5

OSLO, May 21 (Reuters) - Wage talks involving some 7,500 oil drilling workers in Norway broke down on Wednesday, employers and three labour unions said, raising the risk of strike action later this year that could disrupt exploration.

The talks between the Norwegian Shipowners' Association $(NSA)$ and the Safe, Styrke and DSO labour unions will resume at a later unknown date with a state-appointed mediator.

"Unfortunately it was not possible to reach an agreement," NSA lead negotiator Geir Sjoeberg said in a statement.

Under Norway's tightly regulated collective bargaining system, workers are only eligible to go on strike if the mediation also fails. Details of the demands and any wage offers have not been disclosed and are normally confidential.

A broad-based group of onshore industry workers reached a wage deal with employers in March for a 4.4% pay rise this year, although demands in other sectors could deviate.

Strikes by drilling workers could delay expansion projects and the start-up of new fields in Norway, which is a major global exporter of oil and gas, but typically have only a minor impact on daily output.

Annual wages on offshore drilling rigs in Norway range from 743,000 to 942,000 Norwegian crowns ($73,328-$92,968) plus overtime and other benefits, according to the labour unions.

Companies affected by the talks include Transocean RIG.N, Saipem SPMI.MI, Odfjell Drilling ODLO.OL, Archer, Seadrill SDRL.N and others, and were represented in the talks by the NSA.

A separate group of Norwegian petroleum production workers, who are directly employed by companies such as Equinor and Aker BP, reached a wage agreement earlier this month.

($1 = 10.1325 Norwegian crowns)

(Reporting by Terje Solsvik, Editing by Louise Heavens and Elaine Hardcastle)

((terje.solsvik@thomsonreuters.com; +47 918 666 70;))