Swedish teen climate activist Greta Thunberg on Wednesday urged EU leaders to "do as much as possible" to step up the bloc's ambitions to reduce carbon output at this week's summit.

"Of course none of what has been proposed or pushed is in line with the science, but we are still demanding them to do as much as possible... to commit to what they had promised to do" in the 2015 Paris Agreement, Thunberg told AFP in a video interview.

"It is odd how words and actions don't add up," German Fridays for Future activist Lisa Neubauer said of European leaders' climate commitments.

"It appears like the ones who have made the most claims are the ones who back up the most," she added, singling out French President Emmanuel Macron in particular.

Leaders must agree by the end of this year on their 2030 targets for reducing greenhouse gas emissions, and the European Commission has proposed a goal of slashing them 55 percent compared with 1990 levels -- up from 40 percent under earlier plans.

The ultimate aim under Brussels' timetable would be carbon neutrality by 2050.

Now heads of government will have their say, and must hammer out an option palatable to a European Parliament that has already criticised the Commission's plan as not ambitious enough.

On Wednesday, 11 member states including France, Spain and the Netherlands backed reductions of "at least 55 percent" by 2030.

But many countries in the EU's east, notably coal-intensive heavyweight Poland, are reluctant, and Germany -- which holds the EU's rotating presidency -- is keen to achieve unanimity.

"There's a narrative that some countries are leading, and some countries are the bad guys. But what we need to shine a light on is that even those who are seen as leaders are so unbelievably far from doing what is enough," Thunberg said.

Climate charities have warned that reductions of 55 or even 60 percent in EU greenhouse output would fall short of what's needed to keep global warming below the two-degrees-Celsius ceiling agreed in Paris.

Leaders like Macron have "an incredible opportunity and responsibility to take," Thunberg said.

"And if people like him can't take it, if not even countries like France can't take the responsibility, then how can we expect countries like China or India to live up to their commitments?"