FRANKFURT, Germany -- A strike by cabin crew at German airline Lufthansa disrupted hundreds of flights on Friday, leading to long lines of stranded passengers at Germany's biggest airport.

Officials from Lufthansa told NBC News that they were forced to cancel a large share of the airline's 360 flights scheduled for the first half of the day to and from Frankfurt.

The industrial action affected mostly domestic and inner-European connections, but Lufthansa also cancelled flights from Boston, Philadelphia and Atlanta to Frankfurt, the airline said.

Thousands of people are estimated to be stuck or delayed at the Frankfurt airport, according to Reuters.

Frankfurt Airport operator Fraport briefly asked for no flights to depart from European destinations to Frankfurt as a result of the strike.

"Because we were anticipating a problem with aircraft parking positions at our airport, we temporarily -- for about 20 to 30 minutes -- asked other German and European airports to discontinue take-offs of inbound flights to Frankfurt," a Fraport spokesman told NBC News.

"This does and did not affect any transcontinental flights or any aircrafts that are presently in the air," the spokesman added.

Union threatens to extend strike
Among the demands of the UFO union, which represents about two-thirds of Lufthansa's 19,000 cabin crew members, was a 5 percent pay increase and a guarantee that the airline would not outsource jobs. Lufthansa has said it is offering a 3.5 percent raise.

The union called the strike after 13 months of negotiations for higher pay and guarantees on conditions failed to produce an agreement.

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While Lufthansa officials stressed that they wanted to return to the negotiating table as soon as possible, union representatives say that the strike could continue for a long time.

But UFO union head Nicoley Baublies said on Bayerischer Rundfunk radio that the Frankfurt strike may just be the beginning if Lufthansa does not meet its demands.

"It depends on how Lufthansa responds now and how much they try to break the strike and put our people under pressure," UFO union head Nicoley Baublies said on Bayerischer Rundfunk radio, according to The Associated Press. He said the union would decide Friday whether to stage similar strikes again Saturday and whether to expand them to other airports.

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"That's always possible and we will announce it with six hours' notice," Baublies said.

Lufthansa said it would try to place passengers on trains and alternative flights.

"The call to strike action forces Lufthansa to cancel a majority of flights from and to Frankfurt," Lufthansa said in a message to passengers on its website.

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"Subject to cancellation are mostly short- and medium-haul flights. Nevertheless, a small number of long-haul flights will have to be cancelled as well even though they have utmost priority and, wherever possible, shall operate. In general, delays must be anticipated throughout the day," it said.

Costly work stoppage
Like most global airlines, Lufthansa is battling soaring fuel prices, weak demand from cash-strapped passengers and economic slowdown, as well as fierce competition from low-cost carriers such as Ryanair.

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Lufthansa, which operates around 1,850 flights daily, mostly from Frankfurt and Munich, also needs to generate more profit to pay for $21.3 billion of new aircraft on order, according to Reuters.

If the stoppages affect the airline's wider European and global network and could cost it millions of dollars a day in lost revenue. 

The Associated Press and Reuters contributed to this report.

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