"We will shortly announce which flights will not be affected,“ said Ufo boss Nicoley Baublies.
Differences between Lufthansa and Ufo relate to a "transitional care“ clause in workers' contracts which allows them to take early retirement.
The union is seeking to ensure that Lufthansa will not reduce its financial commitment to workers' retirement funds and has given the air carrier until Thursday to accept an offer they have put on the table.
The union rejected on Sunday the airline's latest offer on the transitional care clause.
While a dispute has been bubbling in the background between Lufthana and Ufo for two years this is the first time it has come to industrial action.
As Lufthansa seeks to cut costs in the face of competition from budget airlines such as Ryanair and Easyjet, they have become embroiled in salary conflicts with their workforce.
Meanwhile, since April 2014 Lufthansa pilots have gone on strike thirteen times. The last strike was called off when a court ruled it to be illegal.