Some 262 oil service workers at Schlumberger's Norwegian subsidiary plan to go on strike from 3 December if negotiations with employers over pay and working conditions fail, the Safe trade union said on Thursday.

The negotiations cover drillers, well service crews, divers and other workers at oil industry subcontractors Baker Hughes, Vetco Gray, Weatherford, Oceaneering, Schlumberger and Subsea 7.

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If a strike breaks out on 3 December, it could be escalated in the following weeks to encompass workers at the other firms.

Norway is western Europe's top petroleum exporting nation. A strike would not affect ongoing production of oil and gas but would disrupt other functions such as the drilling of new wells, said Espen Johannessen, Safe's union leader at Schlumberger.

Among the fields that would see an impact were Gullfaks, Statfjord and Valhall, he said, as well as a number of mobile offshore drilling rigs.

Schlumberger and the Norwegian Oil and Gas Association which negotiates with the union on behalf of the oil service firms, were not immediately available for comment.

In total, around 700 workers are covered by the collective bargaining agreement between Safe and the Norwegian Oil & Gas Association (NOG), according to Norway's government-appointed wage mediator, who will oversee the negotiations.

In 2016, a three-week strike among Norwegian oil service workers disrupted the drilling of new wells but did not impact production.

The industry's core production workers, who are directly employed by oil firms and thus not part of the latest talks, settled their wage demands last month following a 10-day strike that cut output and rattled energy markets.

A larger labour union, Industri Energi, said in October it had agreed to a wage deal covering some 6500 oil service workers and would not go on strike.

Safe has said the rights of some of its members have been eroded and it fears this could spread, with the potential consequence that wages in certain cases could be cut by as much as 47%.

Separately on Thursday, the planned escalation of a strike by security guards could close down a quarter of Norway's gas exports to Europe within the next few days, gas systems operator Gassco said.