Ryanair Holdings Plc accuses its German competitor Deutsche Lufthansa AG of raising false environmental concerns about so-called ghost flights to protect its bases at major airports.

Lufthansa claimed European Union slot regulations would force it to operate 18,000 flights without passengers during the winter season, causing unnecessary carbon dioxide emissions. While activists including Greta Thunberg appeared to sympathize, Ryanair said the German airline should sell cheaper tickets to ensure planes are not empty.

“Lufthansa loves crying crocodile tears over the environment when it does everything possible to protect its slots,” Ryanair chief executive Michael O’Leary said in an emailed statement. Retaining unused airport capacity “blocks competition and limits choice at large hub airports”, he said.

A Lufthansa spokeswoman declined to respond to Ryanair’s comments.

Lufthansa, whose airline units include Swiss, Brussels Airlines and Austrian Airlines, last week launched a communications campaign against EU use-or-lose rules that say flights must take place or take-off and departure slots landing will be removed. In an interview with the Frankfurter Allgemeine Zeitung, CEO Carsten Spohr raised concerns about the environmental impact and hinted that he would like to cancel the flights.

In response, the EU said the normal requirement to use 80% of allocated winter slots has been relaxed to 50% due to Covid-19, and airlines can apply for further exemptions if they are justified.

The surge is just the latest sign of tension as European airlines and airports scramble to position themselves in the post-pandemic aviation market. Ryanair has accused Lufthansa of using the 9 billion euro ($10.2 billion) bailout it received from the German government to stifle competition.

While the EU required the German carrier to give up some of its take-off and landing rights at Frankfurt and Munich in exchange for bailing out taxpayers, the rules prevented carriers with existing bases, such as Ryanair and Easyjet Plc , to extend theirs to take advantage of the opening.

This is not the first time that Europe’s leading airline group and low-cost carriers have clashed over the environment. Lufthansa has previously said Ryanair’s proposed fares create artificial demand for flights and unnecessary greenhouse gas emissions.