Pilots at German airline Lufthansa say they will extend a strike over pay that has grounded tens of thousands of passengers and cancelled hundreds of flights.

The ongoing industrial action over a long-running salary dispute with management will be extended until Saturday and will affect all long-haul flights that are set to leave from Germany, the pilots' union Vereinigung Cockpit said on Twitter on Thursday. 

The strike is expected to lead to the cancellation of around 830 flights on Friday, after similar numbers of cancellations on Wednesday and Thursday.

By the end of Friday, an estimated total of 315,000 passengers will have been affected.

Lufthansa's executive management board member Harry Hohmeister said it was "not possible" to meet the pilots' demand for a bigger wage rise.

"The starting point is that we already pay our pilots far better than our competitors," Hohmeister was quoted as saying by AFP news agency.

The striking Lufthansa pilots are demanding a pay rise of an average of 3.66 percent a year, retroactive for the past five years. The company has offered a 2.5 percent wage increase.

Their union says pilots have endured a wage freeze over that time and suffered a "significant loss of purchasing power" due to inflation, while Lufthansa has made billions in profits.

Germany's Bild newspaper said the strike was costing the airline around $10.5m a day.

Lufthansa has been battling a series of walkouts by both cabin crew and pilots over the past two years, as it aims to bring down costs to survive competition from low-cost rivals such as easyJet and Ryanair.

In July, it brought an end to the long-running industrial dispute with cabin crew through a deal on pay and working conditions, including a no-strike agreement and job guarantees until 2021.

Lufthansa group's other airlines - Germanwings, Swiss, Austrian Airlines, Air Dolomiti and Brussels Airlines - are not affected by this week's strike.

Source: News Agencies