Staff responsible for checking passengers and cargo have held an 18-hour warning strike at Germany's biggest airports. Hundreds of flights were cancelled.
German airport association ADV warned that the strike action would "paralyze" the German flight network all day Tuesday.
Frankfurt operator Fraport axed 617 out of around 1,200 scheduled flights at Germany's largest airport.
At Germany's second largest airport, Munich, a spokeswoman said around 100, mainly domestic flights were cancelled.
The coordinated industrial action marked a major escalation in the dispute between unions and employers. There had been walkouts at Berlin's airports last Monday and in Stuttgart, Cologne-Bonn and Duesseldorf last Thursday.
The DBB and Verdi trade unions began their walkout at 2 a.m. local time (0100 UTC) on Tuesday. The action was in support of a pay claim seeking to increase the hourly rate for staff to €20 ($23): "Because employers have shown no willingness to make a better offer, the extension of warning strikes has become necessary," Verdi board member Ute Kittel said.
Officials at Munich airport said that although Verdi called for action there as well, passenger security checks were carried out by a public-sector contractor whose workers would not strike.
Talks between unions and employers over the dispute are due to start again on January 23.
There has been a series of strikes in the German air transport industry in recent years.
jm/se (AFP, Reuters, dpa)