The airline industry agreed Thursday on a framework for reducing its carbon footprint at a UN meeting in Montreal, the first time a commercial sector has voluntarily tackled climate change.

Six years of negotiations culminated in the agreement at the International Civil Aviation Organization's plenary session.

Malaysia's aviation chief Azharuddin Abdul Rahman, who chaired the session, called it a "historic" deal to cap carbon-dioxide emissions at 2020 levels, by 2035.

It "makes air transport the very first major industry sector to take action at the global level on international emissions," said ICAO secretary general Fang Liu, describing the measures as "balanced, pragmatic, and a very positive development."

"You have given the industry the ultimate tools to fight against the climate change," added International Air Transport Association (IATA) director Paul Steele.

The agreement comes a day after record-fast country ratifications of last year's landmark Paris global climate accord curbing greenhouse gases from burning coal, oil and gas met the threshold for it to take effect.

The aviation sector deal was reached by consensus despite reservations by India and Russia, and to a lesser extent China.

It covers a segment of CO2 emissions not dealt with by the Paris accord, which comes into force on November 4: Carbon pollution from jetliners, which total about two percent of global emissions.

Although the number of airline passengers are forecast to double during the long implementation of the agreement, the industry is betting it can reach its stated goal through the purchase of credits to offset the sector's CO2 emissions.

The ICAO also is encouraging greater use of fuel-efficient engines running on biofuels and lighter aircraft materials, and route optimization.

So far, 64 countries representing 80 percent of global air traffic -- including all European nations, the United States, Canada, China and Japan -- have agreed to participate in the so-called Carbon Offset and Reduction Scheme for International Aviation, or CORSIA.

- 2027 target -

The pact will be rolled out in two phases, first on a voluntary trial basis starting in five years, followed by full implementation and mandatory participation in 2027.

At this point, even the agreement's detractors such as India and Russia will be required to adhere to the new rules established under CORSIA.

Only the world's poorest nations, small island states and countries with a small volume of global passenger travel (less than 0.5 percent) will be exempted.

Throughout, key assumptions will be tested and adjustments will be made to the scheme, as needed.

Only international flights are to be counted in the sector's emissions tally, and airlines will be able to buy carbon credits from other sectors.

Immediately following its adoption, praise and criticism poured in.

The Russian delegation at the ICAO meeting said the cap was "unrealistic," adding that Russia was not ready to join the framework.

Aircraft builders, environmental activists and others, meanwhile, joined a chorus of voices praising the accord as a positive step.

"This deal is a decisive step towards the carbon neutral growth of aviation," commented European Transportation Commissioner Violeta Bulc.

US Secretary of State John Kerry called it a "significant" and "historic step." "This measure addresses a growing source of global emissions," he said.

"We are far from the finish line in curbing carbon pollution from international aviation," commented Lou Leonard of the World Wildlife Fund.

"But this is the starting block. It's a foundation we must build on over time," he said.

The ICAO agreed. Its president Olumuyiwa Benard Aliu told a news conference at the close of the plenary session Thursday that "there's a lot of work left to do."