EU leaders agreed Friday to a target of making the bloc carbon neutral by 2050, but the deal was immediately undermined by Poland's refusal to implement the aim.

EU President Charles Michel hailed the agreement struck at a Brussels summit after hours of wrangling as a "strong commitment" that puts Europe in the lead of global efforts to tackle climate change.

"We have reached an agreement on climate change. It is very important, it was crucial, for Europe to show strong ambition," Michel said.

But Poland, which currently relies heavily on coal, demanded more time to switch to green energy -- perhaps up to 2070 according to diplomats -- while others like the Czech Republic pushed the case for nuclear power.

"The European Council endorses the objective of achieving a climate-neutral EU by 2050, in line with the objectives of the Paris Agreement," the leaders said in their formal summit conclusions.

To avoid the embarrassment of the summit collapsing in the face of Polish objections -- on the same day as the COP25 global climate conference in Madrid -- the leaders added a temporary get-out clause for Warsaw.

"One Member State, at this stage, cannot commit to implement this objective as far as it is concerned, and the European Council will come back to this in June 2020," the conclusions said.

Brussels' new leadership, under European Commission President Ursula von der Leyen, launched a "European Green Deal" on Wednesday.

It includes a plan to mobilise 100 billion euros to help countries with the move towards carbon neutrality, but critics from eastern Europe say this is not enough.

Prime Minister Andrej Babis said the Czech Republic alone would need 30 to 40 billion euros ($27 to $36 billion) to achieve carbon neutrality.

And Lithuanian President Gitanas Nauseda estimated the transition would cost two-thirds of his country's GDP.

- Viennese coffee -

There was further compromise on the vexed question of nuclear power. Hungary and the Czech Republic wanted the summit conclusions to explicitly state that nuclear would "play a key role in meeting climate objectives".

The final draft stops short of this, simply noting that some countries "use nuclear energy as part of their national energy mix".

As he arrived, the Czech leader hit out at countries like Austria and Luxembourg for refusing to agree to the EU endorsing nuclear energy as green, and as he left for the night he claimed victory.

"I persuaded the EU leaders today at EUCO that nuclear energy is our way to climate neutrality. For Czech citizens, it means enough of clean energy for decades," he tweeted.

French President Emmanuel Macron, whose country gets more than two-thirds of its electricity from nuclear power, backed the Czechs, saying the UN's Intergovernmental Panel on Climate Change agrees it should play a role.

"For countries that have to get away from using coal, it's obvious they will not be able to move to all renewables overnight," Macron said.

And there was more support from Hungary's Viktor Orban, who insisted that "without nuclear energy there is no carbon-neutral European economy".

- The 'frugal five' -

But Luxembourg Prime Minister Xavier Bettel had been adamant he would not agree to EU money being used to set up nuclear plants.

"We are convinced that nuclear energy is neither sustainable nor safe, and especially when it comes to waste, we still do not know what we should do," he said.

The summit began with a dramatic protest by activists from environmental group Greenpeace who climbed the front wall of the Europa Building summit venue before the leaders arrived and unfurled a "Climate Emergency" banner.

The EU's mammoth 2021-2027 multi-year financial framework (MFF), another indigestible morsel on Thursday's summit menu, was kicked down the road, with leaders set to return to it in 2020.

Finland, which holds the rotating EU presidency, has proposed a seven-year budget based on national contributions amounting to 1.087 trillion euros.

This is equivalent to 1.07 percent of Europe's total GDP, but less than the 1.114 requested by the commission and or the 1.3 percent sought by the EU parliament.

A so-called frugal five -- Austria, Denmark, Germany, the Netherlands and Sweden -- oppose allowing EU spending to swell.

But Finland's proposed cuts would have eaten into plans for the green transition, a border force, the digital economy and defence -- dear to France.afp