Scandinavian airline SAS cancelled Monday a majority of its Swedish and European flights after pilots rejected an improved pay offer and their strike pushed into a fourth day.

SAS said some 230 flights and 27,000 passengers would be affected by Monday's action, adding to the 50,000 travellers who have been stranded since Swedish pilots walked out on Friday.

Flights between Stockholm and Athens, Brussels, Dublin, Frankfurt, Geneva, London and Paris were cancelled on Monday.

The strike began after Swedish pilots based in Stockholm rejected the airline's offer of a 2.2 percent wage increase, insisting on a hike of 3.5 percent, and attempts by a mediator to broker a deal on Sunday failed.

"The Swedish pilot union has turned down a second bid by the mediator, which SAS accepted," said the airline.

The walkout comes during peak travel season, and has hit charter groups hard.

SAS, which is 50-percent owned by the Swedish, Danish and Norwegian states, has said it has not calculated how much money it was losing because of the strike, but financial analysts have estimated it was costing the airline at least $1.2 million (1.1 million euros) a day.

The airline says all the pay and contract demands sought by pilots would result in a 10 percent increase in its costs.

However the head of the pilots union, Martin Lindgren, said "the pilots have helped save billions" for the airline by going along with the 2012 recovery plan that included job cuts, salary reductions and administrative cutbacks.

SAS has come under increasing pressure in recent years from low-cost rivals including Scandinavia-based Norwegian, Europe's third-largest budget airline.

While the airline returned to profit in 2015, it managed net earnings of only 171 million kronor (18.3 million euros, $20.6 million) in the second quarter of this year despite low fuel costs. due to fierce competition and exchange rate swings.