The UFO union said it is urging cabin crew and pursers at Frankfurt and Munich airports, Germany's busiest, to walk out between 6 and 11 a.m. (0400 and 0900 GMT) on Sunday.
It added on Monday that further strike calls "are possible at any point from now on" and accused the airline of waging a power struggle against small, specialized unions.
Lufthansa has rejected talks with UFO in recent months because of doubts over the legal status of its leadership. The union has been engaged in an internal leadership struggle.
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