Environmentalists have hailed the Russian parliament's ratification of the Kyoto Protocol on climate change as a huge step forward.
Putin's backing for Kyoto has been vital
Russia's lower house, the State Duma, voted 334-73 to approve the treaty, meaning enough nations have signed up to bring it officially into force.
"We'll toast the Duma with vodka tonight," a Greenpeace activist said.
However Washington said it still did not intend to adopt the pact, which calls for cuts in greenhouse gases.
"We do not believe that the Kyoto Protocol is something that is realistic for the United States and we have no intention of signing or ratifying it," State Department spokesman Adam Ereli said.
The protocol still has to pass through Russia's upper parliament and be signed into law by President Vladimir Putin.
Both further stages should be a formality, says the BBC's Sarah Rainsford in Moscow, meaning the Kyoto Protocol could get final approval from Russia within the month.
Tipping the balance
Russia's move was hailed by activists. "The entry into force of Kyoto is the biggest step forward in environmental politics and law we have ever seen," said Jennifer Morgan, director of the World Wide Fund for Nature (WWF) conservation group's climate change programme.
But some, like Klaus Toepfer, executive director of the UN Environment Programme (Unep), noted that even with Kyoto up and running, "the goal of stabilising the climate and securing the stability of the planet is however a long way off".
European Commission President Romano Prodi said: "The Kyoto Protocol may not be perfect but it is the only effective tool that is available to the international community."
"The United States should not abstain from the one fight that is crucial for the future of mankind."
The US, world's biggest emitter of human produced greenhouse gases, pulled out of the treaty in 2001. Friends of the Earth International called for "international pressure" to encourage the US and Australia to join.
For Russia, backing the pact is more a political move than an environmental one, our correspondent says.
President Putin agreed to fast-track the ratification of Kyoto in May, when the EU promised to support Russia in its bid to join the World Trade Organisation.
However, some officials have argued it could hinder Russia's economic growth.
Buying and selling
Although it was adopted nearly seven years ago, the Kyoto Protocol could only come into force when ratified by developed nations that account for at least 55% of global greenhouse emissions.
After the US pulled out, that figure could only be reached with the support of Russia, which accounts for 17% of world emissions.
Within 90 days of Russia's ratification, Kyoto signatories must start making cuts that will reduce emissions of six key greenhouse gases to an average of 5.2% below 1990 levels by 2012.
Scientists say greenhouses gases may have to be cut by up to 60%
Countries which fail to meet the targets will face penalties and the prospect of having to make deeper cuts in future.
BBC science analyst Tracey Logan notes that many experts believe that Kyoto will be largely ineffective as the world's two biggest emitters, the US and China, will not cut their outputs.
Although China did sign the protocol, as a developing country it is not yet required to begin reducing emissions.