Lufthansa's flight attendants launched a 24-hour nationwide strike at midnight Thursday, forcing the German carrier to cancel about two-thirds of its scheduled flights.

The strike was the most extensive of three limited work stoppages over the last week after contract negotiations broke down.

The union is demanding a 5 percent pay raise for more than 18,000 cabin staff. Lufthansa is offering a 3.5 percent boost and is calling for a slight increase in working hours.

Lufthansa took out ads in major German newspapers Thursday apologizing for disruptions to passengers and calling the union action "disproportionate."

Effects of the strike will not be clear for hours since the walkout went in effect at an hour when commercial flights are severely restricted due to noise abatement rules.