Representatives of 190 nations are meeting in Montreal this week to decide which course to plot to curb CO2 emissions linked to global warming, as the EU readies to take unilateral action to force airlines to pollute less.

The ICAO assembly meets every three years to refine, harmonize or set new worldwide air travel regulations.

"The big question dominating this year's conference is airlines' increasing carbon emissions," Denis Chagnon, spokesman for the ICAO, told AFP, noting consensus had already been reached on new airline safety and security measures going into the talks.

"But we can't predict or anticipate how negotiations (on emissions) will unfold," he said. "Usually, I can prepare a media release a few weeks in advance announcing any new guidelines, but not this time."

One hotly-contested scenario would impose mandatory emissions cuts on some airlines, or continue a voluntary approach for all unveiled at the ICAO's last meeting in 2004.

Giovanni Bisignani, director general of the International Air Transport Association (IATA), representing the airline industry, on Monday challenged ICAO member states to set a target of 120 million tons of carbon emissions annually through "more efficient infrastructure and better operations."

He proposed offering airlines tax credits to renew their fleets and research grants to develop greener aircraft and fuel, rather than include airlines in the EU carbon trading market.

"The most pressing economic issue is emissions trading. Europe's intention to include aviation in its emissions trading scheme was a wake-up call for everybody," he told reporters.

"But the environment is a global issue requiring a global approach.

"If implemented unilaterally, the result would be legal and diplomatic fights with limited or negative impact on the environment," he added.

Canada, the United States, Australia, China, Brazil and India are said to oppose the EU proposal.

In his opening address Tuesday, ICAO secretary general Roberto Kobeh Gonzalez appeared to critize European powers, saying: "In a world tempted by unilateral action on a national or regional basis, we must not forget that aviation is by definition international and that it can only grow safely and securely through multilateral action."

More than 2.2 billion people flew last year.

Commercial flights currently account for two percent of global greenhouse gas emissions, according to the ICAO.

Despite improvements in most aircraft's discharge, total airline industry emissions have increased dramatically in the past decade as more flights took off due to the emergence of cheap air travel, said the UN aviation body.

Airline industry venting is expected to continue rising to three percent of total carbon emissions by 2050, if left unchecked, the ICAO said, while the global fleet doubles to more than 36,000 by 2025 as projected by US aircraft manufacturer Boeing.

Also, the International Panel on Climate Change (IPCC) recently suggested that, at high altitudes, aerosols in jet engine exhaust deplete ozone in the stratosphere, and thus warm the Earth at two to four times the rate of ground-level CO2.

Airlines were not subject to CO2 emission limits under the Kyoto Protocol -- an international pact to stem global warming.

The ICAO had been tasked with tackling aircraft greenhouse gas emissions, but has so far produced only voluntary guidelines to limit or reduce emissions.

Last year, the European Union threatened to unilaterally impose emissions limits for all commercial jets landing in its territory and force airlines into its carbon trading system, accusing the ICAO of dragging its feet in this area.

A European diplomat, on condition of anonymity, told AFP the plan is still being debated by EU member countries, but could become law as early as 2012.

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