Talks between airline SAS and pilot unions are due to resume on Friday as the parties seek a deal to end a strike that the carrier says is threatening its existence.

A majority of SAS pilots in Sweden, Denmark and Norway walked out on July 4 after negotiations over new collective agreements collapsed. The parties returned to the negotiating table in the Swedish capital on Wednesday.

"I have been dealing with issues in the labour market since the mid-70s and I have had mediation assignments for almost 30 years, and there has never been any mediation or negotiation that has been so complicated," Swedish mediator Claes Strath told reporters outside the venue for the talks late on Thursday.

SAS had been struggling in the face of increased low-cost competition for years before the COVID-19 pandemic heaped pressure on all airlines. On Thursday it said the strike had caused 2,550 flight cancellations, affecting 270,000 passengers and cost the carrier between $94 million and $123 million.

On Friday it had cancelled 177 flights, amounting to 62% of those scheduled, according to flight-tracking platform FlightAware.

SAS, which is trying to implement drastic cost cuts and attract new investors, filed for U.S. bankruptcy protection on July 5.

Shares in SAS were down 8% in opening trade on Friday.

Pilots employed by SAS Scandinavia, a subsidiary of SAS Group, have said they would agree to limited wage cuts and less favourable terms, but SAS has said that concessions offered so far are not enough for it to carry out a rescue plan announced in February.

Unions are also demanding that pilots who lost their jobs during the pandemic are rehired at SAS Scandinavia, rather than having to compete with external applicants for jobs on less attractive terms at recently created SAS Link and Ireland-based SAS Connect.

Pilots at SAS Link and SAS Connect are not on strike.

(Reporting by Anna Ringstrom and Johan Ahlander Editing by David Goodman)