A German union called on air traffic controllers to strike for six hours on Thursday to push for higher wages, threatening flight delays and cancellations during the summer holiday season.

The union said up to 2,500 flights could be affected as it rejected the latest offer from German air traffic authority DFS as "not negotiable".

Germany's biggest airline, Lufthansa, said it viewed the threat of a strike with great concern.

The flagship carrier last year took a hit of almost EUR50 million  (US$71.3 million) from a day-long pilots' strike that led to about 2,000 flight cancellations and stranded thousands of travelers.

"Partially the news does come as a surprise, as there were ongoing hopes that some kind of agreement would be reached," Equinet analyst Jochen Rothenbacher said.

He said while the potential financial impact of a strike was not yet clear, he could imagine the stoppage leading to a low double-digit million euro hit for Lufthansa.

Germany's air traffic controllers have demanded a wage increase of 6.5 percent for 12 months, while DFS has offered a rise of 3.2 percent plus a one-time payment this year and an increase by at least another 2 percent next year.

The union rejected the latest offer, saying it represented a pay cut in real terms and included conditions that had not been part of the negotiation .

The planned strike, scheduled to last from 04:00 a.m. GMT until 10:00 a.m. GMT on Thursday, could be averted if the air traffic control authority called for arbitration.

"I don't think this is the end of the story. I think there is still a real chance that an agreement will be reached," said a Frankfurt-based analyst who asked not to be named.

He said he could imagine that DFS would try to delay the strike until after the summer holidays.

"With DFS being a state-owned organisation, I also think they will do their utmost to avert public criticism," he said.