The strike action in Frankfurt, Berlin and Munich follows a walkout on Friday that left 26,000 passengers stranded.

"It is difficult for the company to cushion the impact. We cannot just get new flight attendants, and the personnel buffer is limited," Lufthansa spokesman Klaus Walther said.

By early afternoon, Lufthansa had cancelled over 300 flights, or more than one in six of its daily total.

Its Austrian Airlines unit, unaffected by the strikes because its staff are on separate contracts, said it was using larger aircraft on routes to Germany to help out its parent.

While Germany has a track record of mostly harmonious labour relations compared with other major European countries such as France, its airlines and airports have been hit by a string of disputes in recent years as companies battle to cut costs to cope with the rise of low-cost carriers, soaring fuel prices, fast-growing Middle East airlines and an air travel tax.

A strike by Lufthansa (LHAG.DE) pilots in 2010 caused more than 2,800 flight cancellations, and earlier this year a strike by airfield staff at Frankfurt airport also hit Lufthansa's business.

U.S. soldier Ron Smith, travelling home to Denver, said it seemed each time he flew through Frankfurt he ended up getting stranded. "This wouldn't happen in the U.S.," he complained.

Trade union UFO, which represents around two thirds of Lufthansa's 18,000 cabin crew, called for eight hours of strikes in Frankfurt and Berlin and 11 hours in Munich on Tuesday.

Talks between Lufthansa and UFO broke down a week ago, having failed to result in an agreement for 13 months, and have so far not resumed.

The union wants a 5 percent pay increase and guarantees against outsourcing and the use of temporary workers.

Lufthansa, which is slashing costs in a bid to boost its earnings by 1.5 billion euros, says it will not improve its offer to raise pay by 3.5 percent in exchange for longer hours.

BIGGER THREATS

UFO head Nicoley Baublies threatened the next step could be full-day walkouts by Lufthansa cabin crew across Germany "if this arrogance continues". He has previously said the union was prepared to strike into the autumn or winter.

Analysts estimate the strikes could cost about 5-10 million euros a day, but Equinet analyst Jochen Rothenbacher warned that sum could jump to as much as 50 million euros if full-day strikes across Germany shut down all flights.

Lufthansa's Walther said the airline was mulling legal steps against the union.

At its main hub in Frankfurt alone, Lufthansa scrapped around 200 flights on Tuesday, with about half of short- and medium-haul flights affected, but also one-third of its more lucrative long-haul flights.

The airline, which normally operates about 1,800 flights per day around the world, also cancelled half of long-haul trips due to depart Munich and expects to cancel about a quarter of short-and medium-haul flights.

By mid-morning, Lufthansa had sent 18,000 text messages to customers' mobile phones to inform them of flight cancellations and delays, and it was supplying stranded passengers at airports with drinks and snacks.

At 1235 GMT Lufthansa shares, which have fallen 17 percent over the past year, were down 1.5 percent at 9.659 euros. (Writing by Harro ten Wolde and Maria Sheahan; Editing by Mark Potter)

• A Permanent Economic Emergency