UN and EU officials have welcomed the US declaration that greenhouse gases are threatening to human health.
An EU spokesman said the announcement showed “a degree of resolve” on the part of President Barack Obama to address climate change.
The US move came as delegates from 192 countries got down to work at the UN climate summit in Copenhagen.
Danish PM Lars Loekke Rasmussen has said the summit is an “opportunity the world cannot afford to miss”.
The US declaration could mean the Environmental Protection Agency (EPA) can order cuts in emissions without the approval of Congress.
Rajendra Pachauri, chairman of the UN scientific network on climate change, said the Obama administration was “showing what it can do, even while legislation is pending”.
“It also sends a powerful signal to Congress. It shows a degree of resolve on the part of the president,” he told the Associated Press news agency.
The environment minister for Sweden – which currently holds the EU presidency – said the outcome of the summit depended mostly “on what will be delivered by the United States and China”.
Andreas Carlgren said he would be “astonished” if US President Barack Obama did not offer further concessions when he arrives at the summit next week.
The BBC’s Mark Mardell in Washington said the US announcement had been expected for some time, but still sends an important signal to leaders attending the summit that Mr Obama is intent on passing legislation to curb emissions.
As the Copenhagen summit opened, Mr Rasmussen told delegates the world was looking to them to safeguard humanity.
He said a “strong and ambitious climate change agreement” was needed.
COPENHAGEN IN BRIEF
“By the end, we must be able to deliver back to the world what was granted us here today: hope for a better future,” he said.
Connie Hedegaard, conference president, said political will to address climate change has never been – and never will be – stronger.
“This is our chance. If we miss it, it could take years before we got a new and better one. If ever,” she said.
UN Secretary General Ban Ki-moon has praised France for taking political leadership in the climate change debate.
He said President Nicolas Sarkozy had been “instrumental in bringing the current stage of the negotiation to where we are now”.
Meanwhile British Prime Minister Gordon Brown has said he wants European leaders to commit to deeper cuts in carbon emissions than previously agreed.
The EU has so far only been willing to increase its emissions target if an international deal is reached at the Copenhagen climate summit.
Mr Brown’s comments come as the UK’s official climate watchdog said a new aviation policy was needed to limit an increase in flights.
The report by the independent Committee on Climate Change said it had discussed ideas like levying extra taxes and issuing flying allowances to reduce air travel.
The main areas for discussion at the Copenhagen summit include:
* Targets to curb greenhouse gas emissions, in particular by developed countries
* Financial support for mitigation of and adaptation to climate change by developing countries
* A carbon trading scheme aimed at ending the destruction of the world’s forests by 2030.
Any agreement made at Copenhagen is intended to supplant the 1997 Kyoto Protocol on climate change, which expires in 2012.