An uneasy alliance between airlines as they face down criticism over carbon emissions has collapsed into a high-profile dispute involving Europe’s biggest network carrier and largest discounter.

Deutsche Lufthansa AG Chief Executive Officer Carsten Spohr lashed out at low-cost carriers including Ryanair Holdings Plc in a Swiss newspaper last weekend, saying their loss-leading fares are stoking demand for needless flights that raise pollution and make the industry an easy target for climate campaigners.

Ryanair Chief Marketing Officer Kenny Jacobs hit back by pointing to Lufthansa’s inferior record on carbon dioxide output and emailing 15 million clients in Germany and Central Europe drawing attention to Spohr’s remarks, which he said highlighted his company’s lower fares and emissions.

“I give our thanks to Lufthansa for making our marketing costs less,” Jacobs said Wednesday in Frankfurt, where the German carrier has its biggest base. “We’re very happy with the comparison.”

Spohr’s interview came amid mounting pressure over the close to 1 billion tons of CO2 airlines spew into the atmosphere each year at a time when high-profile polluters such as carmakers and power stations are starting to clean up their acts. The European Aviation Safety Agency said in June it will grade jets by emissions to help people choose between airlines, while France has unveiled a flight tax it says is partly a response to the sector’s role in global warming.

Speaking with Zurich-based NZZ, Spohr labeled fares as low as 10 euros ($11) “economically, ecologically and politically irresponsible” and partly to blame for making aviation “a target for criticism” in the climate debate.

The spat underscores fault lines between network and low-cost carriers, despite efforts to find common ground via lobby groups such as Airlines for Europe, which Spohr helped found with Ryanair counterpart Michael O’Leary.

No-frills carriers have lower average emissions from their fleets of efficient narrow-body models that usually operate close to full. Wizz Air Holdings Plc led the region last year, emitting 60 grams of carbon per passenger-kilometer flown, only two-thirds of the footprint for British Airways owner IAG SA.

The Wizz number fell to 56.5 grams in May, while Ryanair is seeking a reduction to 60 grams by 2030, according to Jacobs. Comparisons of total emissions are even starker, with Lufthansa jets chugging out 32.3 million tons of carbon dioxide in 2018, compared with Ryanair’s 10 million tons.

Spohr’s comments focused on the continuing success of discount carriers in carving out new markets for city breaks, long weekends and drop-of-the hat trips, at a time when flying for fun is being labeled unacceptable by some environmental advocates.

At Lufthansa, most customers are traveling on business or to visit friends and family, with shorter trips existing to feed traffic onto longer flights. Network airlines say it would be impossible to provide such links with the single-fleet, high-utilization business strategies of the discount sector. Instead they need aircraft spanning the smallest narrow-bodies to thirsty four-engine models.