Norwegian oil dispute set for mediation after wage talks break down

* Fourth round of wage talks to fail

* Strike two years ago firmed oil prices (Adds background)

OSLO, June 4 (Reuters) - Wage talks between oil companies and the largest union representing land-based oil workers in Norway have broken down and are heading to state-appointed mediation, the Norwegian Oil and Gas Association said on Wednesday.

The negotiations over pay and work conditions affect about 5,000 employees working for Statoil (Xetra: DNQ.DE - news) , BP (LSE: BP.L - news) , ConocoPhillips (NYSE: COP - news) and Shell (LSE: RDSB.L - news) , among others.

This is the fourth round of talks involving oil workers to head for mediation. So far talks between the companies and platform workers, oil services employees and those operating onshore supply bases have failed to agree a deal.

"Industri Energi did not accept our last offer and chose to walk out of the talks while Safe, Parat and Negotia asked for more time," the Norwegian Oil and Gas Association said in a statement.

These talks will resume after local unions negotiate their own deals, the Safe union said in a separate statement.

Two years ago about 10 percent of Norway's offshore workers went on strike for 16 days, cutting oil output by 13 percent and gas by 4 percent.

The dispute ended when oil companies threatened a full lock-out and the government stepped in to impose a deal. The strike then pushed oil prices above $100 a barrel.

The first mandatory mediation is scheduled for June 16-17 and unions said that if talks fail they would shut down two ExxonMobil and one GDF Suez (TLO: GAZ-U.TI - news) platform with combined production of about 80,000 barrels of oil per day. (Reporting by Gwladys Fouche; Editing by David Goodman)