Airlines and environmental advocates are calling on the European Union to tweak its rules on airport slots in an effort to cut down on the practice that has raised concerns about pollution.

Slots for landing and departure at the biggest airports are coveted, and airlines must guarantee a high percentage of flights still make it into the skies in order to keep their slots. So as airlines see a pandemic-driven decrease in the number of people booking flights, many are opting to send out near-empty planes in order to keep their slots.

Before the pandemic, airlines had to use 80 percent of their flights to keep their slots, but the EU already cut the minimum to 50 percent to curb the number of largely unfilled flights. But airlines experiencing Omicron’s impact are calling on the bloc to further alter the minimum-use rules.

Brussels Airlines said that it will have to conduct about 3,000 flights this winter to protect its slots if the EU doesn’t make any tweaks. Similarly, German airline Lufthansa said that it will have to send out another 18,000 “unnecessary” flights throughout the winter to safeguard its rights.

“The EU surely is in a climate emergency mode,” activist Greta Thunberg tweeted sarcastically this week, linking to a story about Brussels Airlines making unnecessary flights.

Even if the holidays brought a big increase in passengers, the rest of the winter period could be slow as Omicron surges worldwide.

In the U.S., the Federal Aviation Administration has waived similar minimum slot-use rules through March 26, citing the pandemic. Slots are limited at only a handful of U.S. airports, including Kennedy and LaGuardia in New York and Reagan Washington National outside Washington.

Just last month, when there were still some hopes the pandemic might finally wane, the European Commission confirmed the 50 percent rule but said it would be increased to 64 percent at the end of March.

However, major airlines like Lufthansa, Air France and KLM say they are counting on further flexibility, including further decreasing the threshold level on the time slots.

“More flexibility in the short term is required, not just in summer but also in the current winter schedule,” a Lufthansa statement said. “Without this crisis-related flexibility, airlines are forced to fly with planes almost empty, just to secure their slots.”

KLM agreed.

“So if the remainder of the season is very disappointing, as an airline you can find yourself in the situation of either losing slots because you cancel flights or flying with half-empty aircraft. Both situations are not desirable,” the Dutch company said.

It puts the EU in a bind. On one hand, it needs to ensure that airport slots are open to fair competition, allowing newcomers to vie for them if they are not used sufficiently, and on the other, it wants to keep polluting planes from flying as much as possible.

EU Transport Commissioner Adina Valean last month acknowledged the threat of Omicron to the travel industry, but as of Thursday, she had not announced any new regulations.

Belgian Transport Minister Georges Gilkinet wrote her a stinging letter and was lobbying his EU counterparts to join the initiative and increase pressure.

“The high-level pollution created by these flights runs totally counter to the EU’s climate objectives,” according to the letter obtained by the Associated Press.

The Associated Press contributed to this report.