A top European Union official questioning the sincerity of young climate protesters and joking that they have "Greta syndrome" has sparked outrage in Brussels among politicians and students demonstrating against climate change.

Foreign policy chief Josep Borrell triggered the row by saying he believed school students had been galvanised by teen Swedish climate activist Greta Thunberg without fully realising the costs they would have to bear to ensure a carbon-neutral future.

It "can be called 'Greta syndrome'," Mr Borrell, 72, said during a meeting at the European Parliament on Wednesday.

"It is very good to go out and demonstrate against climate change until you are not asked to contribute to pay for it," he said, according to televised footage that was widely shared on social media.

Mr Borrell expressed "doubt" that the students skipping school to stage Fridays for Future climate protests started by Greta "are aware of what these measures will cost them and if they are willing to lower their standard of living".

Swedish activist Greta Thunberg 

A student climate protest leader in Brussels, Anuna De Wever, 18, has slammed Mr Borrell's comments.

"That a commissioner says that this is impossible is completely ridiculous because he knows that it's not and there is no alternative because then humanity will cease to exist if we don't actually solve the climate crisis," she said.

Speaking during a fresh student demonstration against climate change that drew hundreds of people, Anuna acknowledged that retooling society to reduce its carbon footprint was going to be costly, but said her generation was well aware of that.

"This is going to be really expensive," she said. "Obviously we're willing to pay the price to have a future."

Fridays for Future are worldwide climate protests staged by young students 

Greens describe comments as 'unacceptable'

The Greens grouping of MEPs in the European Parliament also criticised Mr Borrell, tweeting that his comments "are unacceptable for a representative of the EU".

They said "we'll be asking for a full explanation from him next week".

MEP Carles Puigdemont, a Catalan separatist who crossed swords with Mr Borrell when the latter was in his previous post as Spain's foreign minister, asked whether the European Commission shared his view of young climate protesters.

A European Commission spokesman, Eric Mamer, told journalists that the entire EU executive "definitely stands in its entirety behind the European Green Deal which acknowledges and supports the aspirations of young - and less-young - people to combat climate change".

The Commission has made tackling climate change the centrepiece of its action, pledging to spend billions of euros a year towards its Green Deal with the goal of making the EU climate neutral by 2050.