A stranded passenger sleeps during a pilots' strike of German flagship carrier Lufthansa at Munich's airport, December 1, 2014. [Photo/Agencies]

FRANKFURT - Pilots at Lufthansa started a two-day walkout on Monday, their ninth strike this year, forcing Germany's flagship airline to cancel close to half of all scheduled flights and leaving thousands of travellers stranded.

The 1,350 cancelled flights will affect 150,000 passengers and wipe several more millions of euros off the airline's earnings, according to analysts.

Lufthansa and trade union Vereinigung Cockpit (VC) have been at loggerheads for months over an early retirement scheme for pilots that was developed decades ago. The most recent round of talks collapsed on Friday.

"The negotiations failed because we just cannot agree on key points ... Lufthansa basically wants to get rid of the collective agreement and have new colleagues receive no more benefits," VC board member Joerg Handwerg told Reuters TV.

VC, representing about 5,400 Lufthansa pilots, is fighting to keep the scheme, which allows pilots to retire at the age of 55 and still receive up to 60 percent of their pay before regular pension payments start at 65.

Lufthansa has said it made concessions in recent talks, including a 5 percent pay rise, but reiterated it would not accept a demand that new pilots, as well as those already with the company, should be able to retire at 55.

The airline is striving to cut costs as it battles stiff competition from budget airlines and Gulf-based carriers.