PARIS (REUTERS) - Air France said pilots protesting over cost cuts and a new expansion strategy for its Transavia unit gave notice that they may extend their strike until next Friday.

Successive rounds of negotiations have failed to end the walkout, which began on Monday and has forced the carrier to cancel close to three-fifths of flights.

"I regret that our concrete proposals, designed to reassure our pilots, have to date not received a reasonable response,"Air France chief executive Frederic Gagey said in a statement on Friday.

The SNPL union warned on Thursday that pilots at Air France, part of Air France-KLM, could vote to extend a strike "indefinitely" if its demands were not met.

With an estimated 60 per cent of its pilots participating in the strike, Air France said earlier that it expected to operate 45 per cent of its flights on Friday and Saturday.

The company has put the cost of the strike at €10 million to €15 million (S$16 million to S$24 million) a day, which threatens to wipe out the €78 million in pre-tax profit that analysts had on average expected Air France-KLM to make this year.

Air France is trying to boost earnings through Transavia, but pilots are worried that it will use labour from cheaper foreign bases.