BRUSSELS (Reuters) -Air France, its Dutch arm KLM, Norwegian, SAS and several Lufthansa Group airlines are among 20 carriers being investigated by the European Union for potential greenwashing, the companies said on Thursday.

The EU said airlines needed to make clear to what extent claims about tackling aircraft CO2 emissions - whether by offsetting them with climate projects or using sustainable fuels - can be substantiated by sound scientific evidence.

The Lufthansa Group airlines under investigation are Brussels Airlines, Lufthansa, SWISS, Austrian Airlines, Air Dolomiti and Eurowings, the group said on Thursday, while a spokesperson for Air France-KLM confirmed the group was also part of the probe.

Norwegian said in a reply to a Reuters request: "We received the letter, and are now working to familiarize ourselves with the case material and will answer this within the deadline."

Nordic carrier SAS also confirmed it had received a letter from EU authorities.

Industry group Airlines for Europe (A4E) said in a statement that it recognised the importance of clear information about sustainability, but that current regulations in the EU "vary significantly between countries and are still evolving".

It is unclear which other airlines are being investigated. The EU has said it will not disclose the names while investigations are at a preliminary stage.

However, the European Consumer Organisation (BEUC), which alerted the EU to the possible greenwashing, said it targeted the following 17 airlines in its complaint: Air Baltic, Air Dolomiti, Air France, Austrian, Brussels Airlines, Eurowings, Finnair, KLM, Lufthansa, Norwegian, Ryanair, SAS, SWISS, TAP, Volotea, Vueling and Wizz Air.

TAP did not confirm it was part of the probe when contacted by Reuters but referred to a statement by the A4E group of airlines issued in reaction to the EU probe.

The other carriers listed by BEUC did not immediately reply to a Reuters request for comment.

Reuters is seeking comment from the other airlines.

It is not clear how closely BEUC's list matches the EU's list, which contains three more airlines.

(Additional reporting by Andrey Khalip, editing by Frances Kerry)