President Barack Obama has called for the US to become energy independent, saying its reliance on foreign oil and global warming posed threats.
Outlining his energy priorities, he said the country would not be held "hostage to dwindling resources, hostile regimes, and a warming planet".
He called for greater fuel efficiency and an "energy economy" aimed at creating millions of jobs.
He also ordered a review of whether states can set car emission standards.
This challenges a Bush administration decision which favoured a national standard for vehicle pollution.
Meanwhile, Secretary of State Hillary Clinton picked Todd Stern - who took part in the Kyoto Protocol negotiations on climate change from 1997 to 1999 - as her envoy for climate change, the state department said.
Mr Stern, who served under former President Bill Clinton from 1993 to 2001, will be the Obama administration's principal adviser on international climate policy and strategy as well as its chief climate negotiator.
"Containing climate change will require nothing less than transforming the global economy from a high-carbon to a low-carbon energy base," said Mr Stern after Mrs Clinton announced his appointment.
"But done right, this can free us from our dependence on foreign oil and become a driver for economic growth in the 21st Century."
Crossroads of history
At his first White House news conference since becoming president, Mr Obama said he would reverse America's dependence on foreign oil while creating jobs, but warned there was no "quick fix".
OBAMA ENERGY PLAN
Reverse US dependence on foreign energy
Review of decision to block states from setting own emission targets
Orders the transportation department to come up with new short-term rules on how carmakers can improve fuel efficiency
Federal buildings to become more efficient
Double 'green' energy from wind, sun and biofuels over next three years
"We will commit ourselves to steady, focused, pragmatic pursuit of an America that is freed from our energy dependence, and empowered by a new energy economy that puts millions of our citizens to work."
He added: "Now is the time to meet the challenge of this crossroads of history, by choosing a future safer for our country, prosperous for our planet, and sustainable."
Mr Obama ordered the Environmental Protection Agency (EPA) to review its refusal of a waiver which had previously allowed California to set its own - stricter - vehicle emission and fuel efficiency standards.
He said California had taken bold moves in implementing the standards.
Mr Obama said: "The days of Washington dragging its heels are over.
"My administration will not deny facts. We will be guided by them."
His statement that the US would lead on climate change was a clear swipe at his predecessor's sceptical view of global warming, says the BBC's James Coomarasamy in Washington.
Energy efficiency drive
California Governor Arnold Schwarzenegger had asked Mr Obama to reverse the Bush administration's insistence on a single, national standard.
California wants a 30% reduction in motor vehicle greenhouse gas emissions by 2016, achieved by improving fuel efficiency standards.
President Obama also ordered the transportation department to come up with new short-term rules on how carmakers can improve fuel efficiency.
A 2007 law required that new cars and trucks produced by 2020 obtain 35 miles per gallon of fuel (about 15km/litre).
However, then-President George W Bush did not put in place the regulations to enable the law to be carried out.
Car exhaust is a major contributor to greenhouse gas emissions
The new rules Mr Obama wants to put in place would mean the new standard is reached by 2011, the New York Times said.
The president also announced plans to make all federal government buildings more energy efficient, and pledged to cut families's energy bills by "weatherising" 2.5 million homes.
He also said the US would double its capacity for "green" energy generation, from sources such as wind, sun, and biofuels, over the next three years.
More than 3,000 miles of transmission lines would be established to transmit the energy.
In the European Union, a recently agreed climate package set out average emission targets for the whole car industry of 120g of CO2 per kilometre by 2012 for new cars, compared with current levels of 160g/km.
The EU target for 2020 is 95g/km. But CO2 emissions vary from car to car, and manufacturers have been given until 2015 to meet their specific targets for each model.
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