Dutch airline KLM misled consumers with "vague and general" adverts about its efforts to reduce the environmental impact of flying, an Amsterdam court ruled Wednesday in a greenwashing case brought by a pressure group.

KLM also "paints an overly rosy picture of the impact of measures such as Sustainable Aviation Fuel (made from renewable raw materials) and reforestation", the court ruled.

"These measures only marginally reduce the negative environmental aspects and give the mistaken impression that flying with KLM is sustainable."

The case was brought by the Fossielvrij NL (Fossil-free Netherlands) group, which accused KLM of greenwashing -- conveying a false impression or providing misleading information about the extent to which a company's products, operations or services are environmentally sound.

KLM is no longer carrying the adverts in question, so the court did not order any alterations.

The airline "may continue to advertise flying and does not have to warn consumers that current aviation is not sustainable", the court said.

"If KLM informs consumers about its ambitions in the area of CO2 reduction, for example, it must do so honestly and concretely," the verdict added.

Most of the adverts were part of KLM's "Fly Responsibly" campaign, which the airline says is an "awareness campaign".

They range from general statements such as "join us in creating a more sustainable future" to declarations about KLM's use of Sustainable Aviation Fuel (SAF), described as a "promising solution".

In the case of SAF, the court ruled that while it can contribute to reducing the harmful impact of flying, "the term 'sustainable' is too absolute and not sufficiently concrete.

"The statement that it is a 'promising solution' also gives too rosy a picture," according to the court.

According to the court documents, KLM had disputed the idea that the statements were misleading and said the firm was free to communicate about its sustainability efforts.

The firm said in a statement it had not used the expressions at the heart of the case "for some time".

"It is good that the court gives us more clarity on what is possible and how we can continue to communicate transparently and honestly about our approach and activities," said the airline.

"We are pleased that the court ruled that we can continue to communicate with our customers and partners about our approach to making aviation more sustainable. We are continuously learning how best to include them in this."