A four-day oil workers strike in Norway that may escalate on Monday would have to last a month to impact Aker BP’s production from the Valhall field in the North Sea, the Norwegian energy company said on Friday.

Chief Executive Karl Johnny Hersvik said activity at the Valhall IP water injection and Valhall DP drilling platforms would stop if the strike is escalated.

“If the strike is prolonged it will have later in the year impact on when wells are back in production ... if that is going to happen it means the strike has to be quite long,” he said during the firm’s second-quarter results presentation.

Aker BP later told Reuters a strike of a month to a month-and-a-half could impact output at the field by “a few thousands barrels per day”.

“Some of the wells that are drilled are planned to be put in production at the end of this year. If the strike lasts, the output will move to next year which would have impact on this year’s production outlook,” investor relations contact Kjetil Bakken told Reuters.

In the short term, Aker BP plans to use any lost drilling time to perform maintenance and activities to raise the flow of oil and gas, Chief Executive Hersvik said.

Hundreds of workers on Norwegian offshore oil and gas rigs went on strike on Tuesday after rejecting a proposed wage deal.

The strike so far has closed Shell’s Knarr field which produces 23,900 barrels of oil equivalent per day.

Norway’s Safe union plans to have an additional 901 workers on strike from Monday (2200 GMT Sunday), unless the dispute is resolved.

The additional employees work on exploration and production drilling rigs owned by Saipem, Transocean, Songa Offshore, Odfjell Drilling, Archer and COSL, among others.

Aker BP holds a 90-percent stake in the Valhall field, which produced 13.5 million barrels of oil equivalents (boe) in 2017, but the firm has plans to increase production.