That pact would aim to improve on two decades of stuttering cooperation and rein in emissions of carbon dioxide blamed for a disruptive rise in temperatures.
There is broad acceptance for an overall EU goal of cutting carbon emissions from homes, power plants, cars, planes, farms and other sources by 40 percent in 2030 compared to the global benchmark year of 1990.
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But arguments about helping poorer eastern states or preferring nuclear over wind or solar power may drag the negotiations through the night into Friday, diplomats said.
The plan is to build on an existing set of green policy goals to 2020, which have greatly increased the amount of renewable energy such as the sun and wind used across the European Union and cut overall energy use through measures such as better building insulation and more fuel-efficient vehicles.
At present, EU carbon emissions are nearly 20 percent below 1990 - helped by the collapse of polluting industries in its eastern member states after the communist collapse 25 years ago.
After six years of economic crisis, however, many states say their national budgets will struggle to cover the cost of more ambitious targets. And leaders fret about the reaction of voters to a range of related effects, from a loss of jobs in coalmines to new EU regulations limiting the power of domestic appliances.
Coal-dependent Poland, whose new Prime Minister Ewa Kopacz is attending her first EU summit, fears the political fall-out from policy that could shut mines a year before an election.
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Britain's David Cameron, pushing for minimal restrictions on how to meet the target, wants to keep open nuclear options and fears red tape will fuel Eurosceptic demands to quit the EU.
Portugal and Spain, keen to export energy, are pressing hard for stricter targets that would push France to accept more power connections being built to link Iberia across the Pyrenees.
EU officials still reckon an outline deal can be reached, but only after many hours of debate on providing financial incentives for nations such as Poland without placing too big a burden on richer, less industrial countries such as Denmark.
"There is no agreement yet but I think the differences of opinion have been narrowed down to a couple of outstanding issues that will be settled by the leaders on Thursday night," one senior official said.
"What the leaders will do is basically to agree on the world's most ambitious yet cost effective climate and energy policy framework."
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While a pan-EU agreement will commit governments for years to come, detailed legislation will only be worked out later and could take account of how far other global powers follow suit.
The European Commission, the EU executive, has laid out three 2030 targets: as well as cutting emissions by 40 percent from 1990, they propose green fuel should provide at least 27 percent of energy and improving energy efficiency by 30 percent.
Among other issues on their agenda, leaders are likely to endorse more help for African countries to fight Ebola and agree to appoint a coordinator for EU efforts against the disease.
They will also press Russia to do more to do more to defuse the conflict in Ukraine as the Kiev government holds a general election this weekend.
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They are unlikely to make changes to trade sanctions against Moscow but could discuss Kiev's request for new loans as the EU tries to broker a deal to restore supplies of Russian gas to Ukraine before winter sets in.
On Friday, they are due to discuss the troubled economy and proposals from the new EU executive, the European Commission to be headed by Jean-Claude Juncker, to promote investments to stimulate growth and jobs. Members of the euro zone are also due to discuss specific issues facing the common currency area.
But officials expect little in the way of decisions on economic issues, with tension between Germany and France over spending and austerity to be handled in the weeks to come.
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