The European Union's member states have been haggling for months over two separate targets for slashing greenhouse-gas emissions: one for 2035 they must bring to the UN climate talks, and the other for 2040.

"This is a crucial day," French Environment Minister Monique Barbut told reporters as the final negotiations began in Brussels -- warning that turning up empty-handed at the November 10-20 talks in Belem would spell "disaster" for the EU.

Behind only China, the United States and India in terms of emissions, the EU has been the most committed of the major polluters to climate action and has already cut emissions by 37 percent compared to 1990.

But after blazing a trail, the EU's political landscape has shifted right, and climate concerns have taken a back seat to defence and competitiveness -- amid concerns in some capitals that greening Europe's economy is harming growth.

"It takes 27 to tango," summed up the bloc's environment chief Wopke Hoekstra as he headed into Tuesday's talks.

The most urgent challenge for ministers is to reach a unanimous deal on an EU emissions target for 2035, known as a Nationally Determined Contribution (NDC), which Paris Agreement signatories are supposed to bring to the COP30.

"The decision is today," German Environment Minister Carsten Schneider declared on arrival. "I want our heads of state and government to go to Brazil with a very strong mandate, a clear leadership role for Europe."

Beyond the Belem target, the EU hopes to lock in the support of a weighted majority of countries for the next big climate target set out by the European Commission on the path to carbon neutrality by 2050.

The EU executive said in July it wanted to cut emissions by 90 percent by 2040, compared to 1990 levels -- a major step towards net-zero.

But the bloc's capitals have yet to endorse that next step, which would require sweeping changes to industry and daily life at a time of growing concern over adverse impacts on Europe's economy.

Spain and the Nordic countries support the 2040 proposal, as does Germany -- with some caveats. But Hungary, Poland, the Czech Republic and Italy remain opposed, citing risks to their industrial sectors.

Heavyweight France, meanwhile, has kept observers guessing on its position -- demanding guarantees its nuclear sector would not lose out under green transition plans and wiggle room on emissions should Europe's forests absorb less carbon than expected.

- 'Not pretty' -

To win over the staunchest sceptics, Tuesday's talks will cover a range of "flexibilities" for member states, including letting countries count carbon credits purchased to finance projects outside Europe.

A commission pledge for credits to account for up to three percent of a nation's 2040 emission cuts failed to win over hardliners, with countries including France pushing for a higher threshold of five percent.

Some countries also want a clause allowing the 2040 target to be reassessed every two years.

Environmental groups have accused states of undermining the bloc's climate ambitions with loopholes.

But a diplomat involved in the process defended the compromise shaping up in Brussels, while conceding it was "not necessarily pretty".

"In the muddy, messy, nasty real world out there, we are trying to achieve something good," said the diplomat, granted anonymity to discuss the sensitive deliberations.

With the clock ticking down to COP30, EU countries in September agreed to a non-binding "statement of intent" for the bloc to cut emissions by between 66.25 percent and 72.5 percent by 2035.

To avoid deadlock, ministers are expected Tuesday to formalise that range as the bloc's legally binding NDC under the Paris Agreement.

The EU insists it remains committed to its role as global climate leader, having mobilised 31.7 billion euros ($36.6 billion) in public climate finance in 2024, making it the world's largest donor.