Long queues were already forming at the Düsseldorf airport on Monday, with only a few security gates open, an airport spokesman said. To speed up lines, passengers should "do as much as possible without hand luggage," the spokesman said.

Some 53,200 passengers in total were expected to travel through the airport on Monday.

By the late afternoon, 172 flights of the 593 scheduled had been grounded.

The strike, which began at around 4:00 a.m. (0300 GMT) Monday morning, is expected to last until midnight.

"As long as no new offer is on the table, our only choice is to apply maximum pressure," ver.di negotiator Andrea Becker said.

The union, which is seeking pay increase of up to 30 percent for private security workers, held a similar strike at Cologne airport last week.

The union says that 70 to 80 percent of the security staff in North Rhine-Westphalia are in the lowest pay bracket, earning just 8.23 euros ($10.70) per hour. Verdi is seeking raises of between 2.50 euros and 3.64 euros more per hour for its members.

Verdi's Becker said the union hoped a new collective bargaining agreement could be reached before the upcoming Easter holidays.

Meanwhile on Monday, some 600 security workers at Hamburg Airport reached a deal that gave them a 15 percent pay increase, raising their take home pay to 13,60 euros per hour.

kms,dr/rc (Reuters, dpa)