UN Secretary General Kofi Annan described the day as "an historic occasion" after he received documents from Russia which pushed the Kyoto Protocol over a ratification threshold, transforming it into an international treaty with effect from next February 16.

"I think this is a great day for the world. Without controlling the environment and climate change, I don't think we stand a chance of achieving millenium development goals and arresting climate change," Annan said.

"Now we need to intensify our efforts to reduce greenhouse emissions. I hope that even those countries that have not ratified yet will make efforts in that direction."

Under Kyoto, only industrialised countries are required to meet specific, binding targets for curbing output of carbon gases that build up in the atmosphere, trapping solar heat.

Developing countries, for their part, pledge to combat global warming and are being offered financial incentives not to follow the same polluting path as industrialised nations.

Critics say this is a major loophole, as fast-growing populous countries like China -- now the second biggest single polluter after the United States -- and India are excluded from any binding commitment.

The big industrialised holdouts from Kyoto are Australia and the United States, which together account for 30 percent of global greenhouse-gas pollution.

Joke Waller-Hunter, executive secretary of the UN Framework Convention on Climate Change (UNFCCC), which oversees Kyoto, mentioned those countries by name and urged them, and other big polluters, to address carbon pollution urgently.

Russian ratification meant a "a period of uncertainty has closed," she said.

"Climate change is ready to take its place at the top of the global agenda," she said.

"Reducing the risks of global warming will require the active engagement of the entire international community," Waller-Hunter added. "I urge the US and other major emitters without Kyoto targets to do their part by accelerating their national efforts to address climate change."

This year's Nobel Peace laureate, Wangari Maathai, a Kenyan environmentalist, agreed that a shadow had been lifted from Kyoto.

"This action today by the people and government of Russia puts the protocol at a completely different position and we hope that indeed we have created a better world for tommorrow," she said.

Russia's ratification was necessary for Kyoto to survive.

Its ratification clauses require a minimum threshold of approval by polluting industrial signatories for it to be transformed from a draft agreement into a full-fledged treaty.

Russian President Vladimir Putin signed the protocol on November 5, just over a week after his country's parliament voted to ratify it.

Russia's response on Thursday focussed on the financial windfall that the country hopes to gain under Kyoto's market mechanisms.

Russia will easily meet the 2008-12 timeframe of making emission cuts because so many of its polluting plants were closed down after the collapse of the Soviet Union in the early Nineties.

That means it has a lot of emissions to sell under Kyoto's proposed carbon market. Countries that are under their pollution target can sell the rest of their quota to nations that are over it, thus providing an incentive for cleanliness.

Putin was quoted Thursday by the UN Environment Programme (UNEP) as saying Kyoto was vital for "the promotion of international cooperation."

It will have consequences for "Russia's social and economic development", Putin said.

Andrey Denisov, Russia's ambassador to the UN, who took part in Thursday's ceremony, said ratification "will have great socio-economic consequences to my country."

"I believe that this step will be helpful for developing good international cooperation in preventing the emission of greenhouse gases, which is important to mankind."

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