Russia handed official ratification papers to UN Secretary General Kofi Annan in Nairobi on Thursday.

Russia's accession gives Kyoto support from countries that emit at least 55% of the world's greenhouse gases.

The protocol commits 55 industrialised nations to making significant cuts in the emission of gases such as carbon dioxide by the year 2012.

Moscow's ambassador to the UN, Andrei Denisov, handed Russia's accession papers to Mr Annan in Nairobi, where the Security Council is holding a special session.

Mr Annan described Russia's accession to the Kyoto Protocol as a "historic step forward in the world's efforts to combat a truly global threat".

The formal ratification of the protocol ended years of uncertainty over the future of the agreement, Mr Annan added.

International disagreement

The US, the world's largest emitter of greenhouse gases, withdrew from the protocol in 2001, saying it would gravely damage the US economy.

The Bush administration also criticised the protocol for not forcing developing nations including India and China to cut emissions immediately.

Australia, which has a large coal industry, has also refused to ratify Kyoto.

The protocol was first agreed in 1997, but required the agreement of countries responsible for at least 55% of global emissions measured in 1990.

After the US pulled out, the protocol could not be ratified without Russia, responsible for 17% of emissions.

Industrialised countries will have until 2012 to cut their collective emissions of six key greenhouse gases to 5.2% below the 1990 level.

But some experts have claimed that a drastic cut of around 60% is needed to avoid the worst effects of global warming.