BERLIN (Reuters) - A scheduled strike by flight attendants protesting against pay and working conditions at German airline Lufthansa on Sunday will now extend to staff at all the company's German units, cabin crew union UFO said on Friday,

The union called on cabin crew at the German units - including Germanwings, Eurowings, SunExpress Deutschland and Cityline - to join the walkout alongside their colleagues at the mother airline.

UFO earlier this week announced a strike for Sunday targeting Lufthansa's namesake core brand at its most important hubs, Frankfurt and Munich.

"We regret that this escalation has become necessary but see no other way of achieving our justified demands," said UFO official Nicoley Baublies in a statement. Lufthansa did not immediately comment.

Some 500 flights are expected to be affected by the walk-out, which will affect all departures from German airports between 0500 and 1100 local time, UFO said.

Germany's biggest airline has called the planned strike illegal, adding that it would try to offer its full flight schedule.

(Reporting by Thomas Seythal and Klaus Lauer, editing by Thomas Escritt)