Unionised pilots at South Korea's Asiana Airlines Inc began an indefinite strike on Sunday after talks with management failed, forcing the airline to cancel 37 percent of domestic flights scheduled for Monday.
Asiana, South Korea's No.2 carrier, had said it would rely on non-union pilots to continue normal operations, but it had to cancel two domestic flights scheduled on Sunday.
The disruption comes at the start of the busy summer holiday season.
"All efforts to reach agreement with the company have failed," said Lee Sang-joon, a spokesman at the 527-member pilots' union. "We began the strike from noon (0300 GMT) as planned."
Asiana held a round of negotiation on Sunday with union leaders seeking better working conditions but could not reach agreement, a company spokesman said.
The two sides are to meet for further talks on Monday.
Some 64 percent of Asiana's 826 pilots are unionised. The union belongs to the militant labour umbrella group, the Korean Confederation of Trade Unions.
The airline plans to cancel 62 of 168 domestic flights and four of seven cargo flights scheduled on Monday. Asiana said the strike would not affect 115 international flights due on Monday.
It was not immediately clear whether flights scheduled for later in the week would be affected if the strike continues.
"The company will rely on non-union pilots and union members who did not join the strike," Asiana said in a statement. "The priority goes to international flights and Seoul-Cheju route over cargo and other domestic flights."
The Asiana union staged a one-day strike earlier in July over job security and better working conditions, including more time between flights. The company had said many of their requests were unacceptable.
Separately, the pilots' union at the country's top carrier, Korean Air Co, said on its web site that its 26 union leaders would stage an indefinite strike from midnight (1500 GMT) in a symbolic move to increase pressure on management negotiators. About 70 percent of Korean Air's roughly 1,900 pilots are unionised.
Korean Air said no flights would be affected by the union leaders' job action.
The strikes come as pilots at some of the world's top carriers have accepted, or have been asked to accept, large wage cuts and concessions from airlines seeking to slash costs to be competitive with other global carriers.
Labour is the largest fixed cost for airlines, which have also absorbed rising costs for their second biggest expense, jet fuel, which has hit record highs recently.