The US has ruled out any possibility of rejoining the Kyoto Treaty to tackle global warming.
It signed the treaty under Bill Clinton's presidency but President George W Bush refused to ratify it, saying America would pursue its own plans to become more energy efficient.
The US has been defending its decision not to take part in the protocol, just two months before the international agreement comes into force.
Speaking at a UN conference on climate change in Buenos Aires, senior US negotiator Harlan Watson attacked the treaty as being politically motivated rather than based on science.
Russia's ratification last month gave the protocol the final stamp of approval needed to go into force on 16 February.
It commits 39 industrial nations and territories, including Japan and Europe, to trim their output of six greenhouse gases, especially carbon dioxide, by at least 5.2% by 2012 compared with 1990 levels.
