* Union asks tower staff to join strike for six hours on Wednesday
* Strike resumed at 2000 GMT Sun, to run until Thurs 0400 GMT
FRANKFURT Feb 28 (Reuters) - German trade union GdF, which represents around 190 striking workers at Frankfurt airport, said on Tuesday it wants control tower staff to join the walkouts, a move which could increase disruption caused by a strike which has forced the cancellation of 1,400 flights.
The union wants 12 air traffic controllers to join the strike from 0400 GMT to 1000 GMT on Wednesday, which would affect at least 300 flights, GdF board member Markus Siebers said.
"The step is meant to show solidarity with the GdF members in the struggle against Fraport," the union said in a statement.
A strike threatened last year by tower workers across Germany would have disrupted around 3,000 flights a day, including those flying over Germany, not only taking off and landing there.
Fraport was not immediately available for comment. When the union warned late on Monday it was considering extending the strike, airport operator Fraport called on the German government to stop air traffic controllers -- who work for a government-owned company -- from joining the strike.
"The threat to extend the strike would ... lead to economic damage that can under no circumstances be justified," Fraport said in a statement.
Walkouts by the employees who guide planes to parking places started on Feb. 16, but have so far had a limited impact at Frankfurt, Europe's third-busiest airport as measured by passenger numbers.
Fraport has been able to fill the gaps by reshuffling its workforce, meaning around 80 percent of daily scheduled flights have run. The airport averages 1,300 flights a day.
Airline Lufthansa, which accounts for around half of the airport's take-offs and landings, said it was cancelling around 140 flights on Monday and a similar number on Tuesday.
The latest walkouts began on Sunday at 2000 GMT and are due to run until 0400 GMT on Thursday.
Each day of strikes costs Fraport around 1 million euros ($1.4 million) in lost revenue.
The union wants higher pay and shorter hours for around 190 staff who work on the airport apron. Fraport says the demands, which would equate to pay increases of 50-70 percent, are unreasonable.
The walkouts by staff who make up just 1 percent of Fraport's workforce have brought calls for changes to Germany's labour laws to prevent such small groups of striking workers from exerting such pressure.
Shares in Fraport and Lufthansa fell on Monday, with Fraport down 1.2 percent and Lufthansa, additionally burdened by high oil prices, down 3.6 percent.