Updated 10.10pm

Lufthansa's pilots' union said it rejected the airline's latest pay offer, only hours after it was made, but added it has no plans now for further strikes beyond tomorrow.

While not ruling out further action, the union said that any strikes would be announced 24 hours in advance.

Lufthansa said earlier today it had offered to increase wages by 4.4 percent in two instalments, as well as a one-off payment equal to 1.8 months' pay.

The union said the offer was not new as it had already been proposed to workers two months ago and it dismissed Lufthansa's statement as a "public relations move". It wants an average annual pay increase of 3.7 per cent for 5,400 pilots in Germany over a five-year period backdated to 2012.

This week's walkout -- the 14th since early 2014 -- has already grounded some 2,600 planes and affected more than 315,000 passengers. Lufthansa has said the disruption has started to hit medium-term bookings.

Pilots had been threatening to extend the strike beyond Saturday, raising concerns among investors at the growing cost to the company and the wider impact on Europe's largest economy.

Lufthansa says it has to cut costs to compete with leaner rivals such as Ryanair on short-haul routes and Emirates on long-haul flights, despite making a record profit in 2015.

Lufthansa pilots are well paid by industry standards. A pilot at Lufthansa earns on average €180,000 ($190,000) a year before tax, though a captain on the highest pay level can earn as much as €22,000 a month before tax.

It is the 14th time Lufthansa pilots have gone on strike since 2014. Photo: Reuters/Ralph Orlowski

Long-term damage?

Passengers at Frankfurt airport today Friday were running out of patience.

"It's not pleasant," said Lufthansa passenger Dieter Eidt who was booked on a flight to Rome and faced a lengthy delay.

"I believe that they are demanding something that can't be fulfilled and which is unjustified," Eidt said of the pilots.

Lufthansa has put the cost of the stoppage at around €10 million ($11 million) per day. However, the airline could take a longer-term hit if the strike prompts customers to shun Lufthansa and switch to rival airlines.

Travel search firm Kayak said nine percent fewer visitors to its site on Wednesday and Thursday had opted for a Lufthansa flight compared to the previous week.

"Many people are looking for alternatives," said Julia Stadler-Damisch, regional director at Kayak. But she said she did not expect the airline's reputation to suffer in the long run.

Investors were not so sanguine.

Shares in Lufthansa, down more than 13 per cent this year, slipped lower on Friday as investors fretted over the prospect of stoppages dragging on.

"The name Lufthansa stands for safety, punctuality, reliability," said Michael Gierse, a fund manager at Lufthansa shareholder Union Investment. "One has to wonder how much of the last two values still remain after the strike."