Russian ratification would, due to a complex weighting system, virtually ensure the treaty is implemented despite its rejection by the biggest air polluter, the United States.

"Russia has signed the Kyoto Protocol and we are now preparing its ratification. We consider that ratification will take place in the very nearest future," Kasyanov said to applause from a plenary session of the U.N. meeting.

The treaty has been passed to the Russian parliament.

European Union nations in particular are pressing Russia to have its parliament ratify the 1997 treaty as soon as possible to bring it into effect and open the way to special aid flows for poor countries hit by climate change.

Shortly after his speech to the summit, Reuters asked him whether he expected the ratification to take place this year.

"Maybe this year," he replied, but declined further comment.

China, the world's second biggest polluter, earlier told the U.N. meeting it had ratified the agreement. But as a developing country, China is not bound by any goals for restraining emissions of carbon dioxide, mostly caused by burning fossil fuels such as oil and coal.

Targets under Kyoto so far apply only to developed states but might in future be extended to China, the world's most populous nation with more than a billion people.

"I would like to announce hereby that the Chinese government has ratified the Kyoto protocol," Premier Zhu Rongji told delegates at the World Summit on Sustainable Development in Johannesburg.

China had been expected to back the Kyoto climate pact.

The agreement holds industrialised nations to cutting emissions of carbon dioxide to around five percent below 1990 levels by 2012.