BERLIN (Reuters) - German pilots' union Vereinigung Cockpit said on Thursday it wanted more information from Lufthansa before it could decide whether to revive failed pay talks, meaning more strikes could be on the cards at any time.

Lufthansa pilots have been out on strike for six days in total since the latest round of protests started on Wednesday last week, grounding 4,460 flights and affecting more than 525,000 customers at one of Europe's largest airlines.

To halt the strikes, Lufthansa said this Wednesday it had dropped demands for pilots to work longer hours in exchange for a wage increase. It is offering to raise pilots' pay by 4.4 percent in two installments in 2016 and 2017 and make a one-off payment worth 1.8 months' pay.

But the union said it had not received a formal notification from Lufthansa about the one-off payment, nor was it clear whether the demands for concessions in exchange for higher pay were really off the table.

"VC expects a firm and clear offer in order to be able to determine whether to resume talks, with a mediator or not," it said in a statement on Thursday.

A spokesman for Lufthansa said the carrier would discuss the points raised with the union directly, promising to do so "promptly".

The dispute over pay involves contracts dating back to 2012 and has seen 15 strikes since early 2014, costing the airline hundreds of millions of euros in lost profit.

Lufthansa has tried to stand firm despite the ongoing strikes, saying it has little choice other than to cut costs if it wants to compete with leaner rivals such as Emirates on long-haul and budget carriers on short-haul flights.

Management has already agreed to pay and pension reforms with cabin crew and ground staff. Some ground staff staged their own protest against the striking pilots on Wednesday, saying they feared the pilots were endangering jobs at the company.

VC has called for an average annual pay rise of 3.7 percent for 5,400 pilots over a five-year period backdated to 2012.

The airline has estimated the immediate costs of the latest walkouts at about 10-15 million euros a day, although the impact on future bookings is not yet known.

Lufthansa returned to a normal schedule on Thursday, though 40 flights were canceled as the airline recovered from a strike in the previous two days.

Research by travel search engine Kayak.de showed searches for Lufthansa flights were down by 9 percent from Nov. 23-28 when compared with the previous week.

(Reporting by Victoria Bryan; editing by Harro ten Wolde and David Clarke)