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

Lufthansa has claimed European Union slot regulations will force it to fly 18,000 flights without passengers over 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 make sure the planes aren’t empty.

“Lufthansa loves crying crocodile tears about the environment when doing everything possible to protect its slots,” Ryanair Chief Executive Officer Michael O’Leary said in an emailed statement. Holding onto unused airport capacity “blocks competition and limits choice at big hub airports,” he said.

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

Lufthansa, whose airline units include Swiss, Brussels Airlines and Austrian Airlines, last week launched a communications onslaught against the EU’s use-it-or-lose-it rules that stipulate flights must take place or the takeoff and landing slots will be taken away. In an interview with the Frankfurter Allgemeine Zeitung, CEO Carsten Spohr raised concerns about the environmental impact and implied he’d like to cancel the flights.

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

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

While the EU demanded the German carrier surrender some of its take-off and landing rights at Frankfurt and Munich in exchange for the taxpayer rescue, rules prevented carriers with existing bases, such as Ryanair and Easyjet Plc, from expanding theirs to take advantage of the opening.

It’s not the first time Europe’s leading airline group and low-cost carriers have clashed over the environment. Lufthansa has previously said the fares offered by the likes of Ryanair create artificial demand for flying and unnecessary greenhouse gas emissions.