BERLIN (AFP) - Pilots at Germany's biggest airline Lufthansa said they would go on strike Tuesday - for the fourth time in three weeks - over changes to early retirement rules, a union said.

The eight-hour walkout will go from 9am to 5pm local time and hit long-haul flights, the pilots' union Cockpit said in a statement on Monday. No flights will take off from Frankfurt, the airline's biggest hub, it added.

"As Lufthansa management has not put forward an offer that is suitable for a compromise, we see ourselves forced to take further action," it said. "We regret any inconvenience to Lufthansa customers."

The airline has been hit by several rounds of industrial action over its plans to change the early retirement policy. A strike last Wednesday grounded 110 flights across Europe, affecting some 13,500 passengers.

The three walkouts taken together have forced the cancellation of 475 flights in total, almost entirely on domestic and European routes.

Due to advance notice, the industrial action has not sparked chaos at German airports. But it has dealt a heavy blow to Lufthansa, which already incurred losses of around 60 million euros (S$98.2 million) from three days of strikes in April.