France and other countries have pushed to have the topic, which is not officially on the summit agenda, added to it -- with talks likely to take place over dinner, according to several European diplomats.

The EU has set a goal of becoming carbon neutral by 2050 and says it has already cut emissions by 37 percent compared to 1990.

Brussels now needs to agree on an interim target for 2040 -- expected to be unveiled on July 2 -- but it faces a tough balancing act.

The European Commission said last year it aimed to cut emissions by 90 percent compared to 1990 levels by that date, to tackle rising temperatures that are wreaking havoc at an alarming pace with wildfires and floods.

But political winds have since turned, pushing it to switch focus to boosting European industry confronted by fierce competition from the United States and China.

For some countries, including the Czech Republic and Italy, the 90 percent target is unrealistic, while France has warned that Brussels should refrain from setting targets without detailing how to meet them.

"We need to ensure that the conditions for preserving the competitiveness of industry and decarbonising industry are in place," said an adviser to French President Emmanuel Macron.

"This means funding... protection for sectors facing unfair international competition, and the fight against carbon leakage," the adviser said, referring to the situation where climate rules in one region result in lower emissions there but a spike elsewhere, as companies seek out laxer jurisdictions.

The commission is considering greater flexibility in its calculations for 2040, including through the purchase of carbon credits on international markets.

But the move is opposed by green groups that say it would allow the EU to outsource part of its climate effort, and worry about a lowering of the bloc's mid-term goals.

"President Macron risks derailing Europe's climate ambition," said Sven Harmeling, head of climate at CAN Europe, an environmental organisation.