Adds union quote, detail in paragraphs 1-2, 5-11

OSLO, June 3 (Reuters) - Norway's oil and gas output could initially fall by about 45,500 barrels of oil equivalent per day from Friday if workers go on strike, equivalent to just over 1% of the country's output, industry group Offshore Norway said on Wednesday.

Nearly 8% of Norwegian offshore oil and gas workers plan to strike from June 5 if state-brokered wage mediation fails in the coming days, with the option to escalate the action over time, three labour unions have said.

Norway produces more than 4 million barrels of oil equivalent per day, almost equally divided between crude and natural gas, and any reduction could impact markets at a time when Middle East output is curtailed by the Iran war.

The three unions, Safe, Styrke and Lederne, have around 8,100 members involved in oil production. Of those, just over 600 would take part in an initial wave of strikes if talks break down, they said.

"Our members occupy important positions on offshore installations, and the strike can have an extensive impact on production," a Lederne spokesperson told Reuters ahead of the talks on Wednesday.

The fields and platforms that would initially be affected by a strike are Statfjord A, Ula, Draugen and Ekofisk 2/4 K, as well as Oseberg B, C and East, the unions said.

The output reduction would involve operators, Equinor EQNR.OL, Aker BP AKRBP.OL, Okea OKEA.OL and ConocoPhillips COP.N, Offshore Norway said.

The industry group declined to provide further details on the production impact.

The labour unions are seeking pay rises above inflation and other changes to contracts, but have not disclosed further information on their demands.

The negotiations between Norwegian oil companies and the labour unions cover most workers on Norway's oil and gas installations.

The labour ministry can intervene to stop a strike if it deems there are exceptional circumstances or that vital national interests are at stake.