Striking Air France pilots dug in their heels Wednesday, rejecting government calls to go back to work and new assurance from management that low-cost operations would not threaten their jobs.
And the pilots' main union has now said the strike may go on for a second week unless their demands are met.
"We've planned a referendum on whether to continue the strike. We'll know on Saturday." SNPL pilots' union head Jean-Louis Barber told French daily Le Monde. "The response is going to be very, very clear. If the pilots vote to continue the strike, this time it will be for an unlimited duration."
The work stoppage was set to go into a fourth day on Thursday, grounding more than half of Air France's flights and costing the company an estimated €10-15 million ($13-19 million) a day.
On Thursday, the airline hopes to put on a similar number of flights -- around 42 percent of planned trips -- director of operations Catherine Jude said.
Prime Minister Manuel Valls told French radio: "We have to stop this strike," which he said was "incomprehensible" to most French people.
"It is regrettable that a single category of employee, in this case pilots, can bring air transport in the country to a standstill," he said.
Economy Minister Emmanuel Macron agreed, saying: "We can no longer accept that the country is blocked by a few people." But he also urged the company to make "a few concessions".
Pilots have walked off the job in protest at the airline's plans to expand its low-cost Transavia business in a bid to compete with cheaper rivals such as Ryanair and Easyjet.
Unions fear the company will seek to cut costs by running more low-cost Transavia flights, where pilots are paid less, at the expense of Air France operations.
Pilots are also worried that the plans will entail job losses as the company sets up bases outside of France and hires pilots locally.
In a letter emailed to the 3,900 pilots on Wednesday, the bosses of Air France-KLM and Air France wrote: "It's not a plan to relocate or outsource operations."
The airline plans to develop Transavia's French, Dutch and European units equally, Alexandre de Juniac and Frederic Gagey said in the letter seen by AFP.
The unions already rejected an offer floated by Gagey on Tuesday to increase Transavia France's fleet to 30 planes by 2019, rather than the original target of 37.
In Wednesday's letter, management went a step further, proposing an "immediate" interim measure whereby Transavia Europe would not fly to Paris, Lyon, Nantes and Toulouse, all earlier identified as part of the expansion of Transavia France.
But Julien Duboz, spokesman of the second-largest pilots' union SPAF, rejected the proposal as "unacceptable", saying management was merely "kicking the can down the road" while the risk of shipping jobs out of France remained real.
'Our struggle is just'
Some three in five pilots observed the strike, according to management, while the unions estimated participation at 75 percent. If the strike lasts a week, as the pilots initially threatened, it would be the longest since 1998.
The striking pilots, more than 250 of whom reportedly attended a general assembly on Wednesday, also fear Transavia will cut into the market share of Air France, whose short and medium-haul routes are losing money.
The head of the largest union, SNPL AF, told AFP after the meeting: "The profession is behind us, we have strong mobilisation. That shows that our struggle is just."
An Air France source told AFP that the hourly cost of employing a Transavia France pilot was some 40 percent less than a pilot at the parent group.
A pilot for the French flag carrier earns about €75,000 a year on average, while captains of long-haul flights can earn up to €250,000.
Unions want to ensure Air France pilots are at the controls of any plane with more than 100 seats, regardless of the operating company -- Transavia included.
Airports were largely deserted on Wednesday as Air France notified passengers in advance that their flights were cancelled.
Aviation's loss appeared to be rail's gain, however, with national rail company SNCF reporting a three-percent rise in passenger traffic and putting on extra capacity in response.
One disgruntled passenger at the airport in the southeastern city of Nice, who gave his name as Jean-Pierre, found his domestic flight to western France cancelled.
"It's a bit of a swindle when you see how much the pilots earn," he complained.