* Pilots to strike April 2-4 in retirement row
* Union says Lufthansa did not take up its suggestions
* Dispute covers pay and retirement conditions
* Strike could cost Lufthansa 30-50 mln euros (Adds Lufthansa comment, fresh union comment, estimate of how much strike could cost)
By Victoria Bryan and Peter Maushagen
FRANKFURT, March 28 Pilots at Lufthansa will hold a three-day strike next week in a row over retirement conditions, their union said on Friday, bringing more disruption to travellers after a spate of industrial action at German airports.
The pilots' strike, which will run from 2200 GMT on April 1 until 2159 GMT on April 4, is likely to cause the cancellation of hundreds of flights, and will be the third strike to hit Frankfurt airport, Europe's third-largest hub and Lufthansa's home base, in six weeks.
Lufthansa, Germany's largest airline, said it was considering its legal options but had not yet taken a decision on whether to launch action to prevent the strike.
It called on the pilots to return to the negotiating table and said that while it still hoped to reach an agreement, it was working to minimise the impact of the strike for its customers.
"We regret that during April 2-4 we will probably not be able to bring our customers home or to their destination as planned," the airline said in a statement.
The strike will also affect Lufthansa's cargo arm and its low-cost carrier Germanwings.
If it goes ahead as planned, the strike could be the largest faced by Lufthansa, a spokeswoman said. A four-day strike called by pilots in 2010 was halted after the first day. Analyst Juergen Pieper of Metzler estimated next week's three-day stoppage could cost Lufthansa 30-50 million euros ($41-$69 million) in earnings.
The dispute is over so-called 'transition contracts' awarded to pilots retiring before the legal pension age. Lufthansa is also negotiating with the pilots on a pay agreement for the contract period from May 2012.
The airline introduced the transition contracts around 40 years ago to make up for the gap left between the age at which pilots were forced to retire and the age at which legal pension payments kicked in.
However, pilots can now work until the age of 65, the same as the legal retirement age for airline workers in Germany. Lufthansa argues such transition contracts therefore are no longer needed and wants to get rid of them, against the wishes of the pilots' union.
Union Vereinigung Cockpit (VC), which represents around 5,400, or nearly all, Lufthansa pilots, said on Friday it had tried to offer suggestions to resolve the row but that the airline had not taken up the offer seriously.
More than 99 percent of the pilots had voted in favour of strike action over the transition contract issue.
Lufthansa said it had improved its pay offer and had made a new proposal that would still allow pilots to retire early. It also said it was withdrawing a plan to link pilots' wages to the profit performance of the company.
"We are ready to continue talks at any time ... If everyone involved shows good will, we should be able to avoid a strike," Bettina Volkens, Lufthansa board member for personnel, said in a statement on Friday.
VC board member Ilona Ritter told Reuters the issue here was not pay, but the retirement benefits. The new retirement conditions offer from Lufthansa still represented a worse deal than the present arrangement, she said, especially as it would not apply to new staff joining from 2014.
"We want all pilots to be treated the same," Ritter said, adding that VC would not reject any new date for talks.
The union said it would not strike during the Easter school holiday period, which starts on April 14 across most of Germany.
Germany's robust labour market has fuelled expectations that negotiated wages will rise more steeply and different groups of workers have been using strike action to pressure employers.
Lufthansa had to cancel a third of its 1,800 or so scheduled flights on Thursday due to a strike by public sector workers employed in areas such as baggage handling at German airports.
At the end of February, industrial action by security staff at Frankfurt airport had caused thousands of passengers to miss their flights. ($1 = 0.7278 euros) (Editing by Erica Billingham and Susan Fenton)