US President Barack Obama is doubling down in the fight against climate change. With Congress refusing to budge, he is poised to unveil sweeping new limits on carbon emissions.

On Monday, the Environmental Protection Agency (EPA) is expected to unveil proposals for drastic cuts in carbon emissions from power plants, which account for 40 percent of greenhouse gas emissions in the United States.

"The shift to a cleaner energy economy won't happen overnight, and it will require tough choices along the way," Obama said Saturday in his weekly radio and Internet address.

"But a low-carbon, clean energy economy can be an engine of growth for decades to come."

Even as natural gas gains in popularity, coal remains a key component in the American energy landscape. Wyoming leads the pack of 25 states that mine the fossil fuel, followed by West Virginia, Kentucky, Pennsylvania and Illinois.

Hundreds of coal-fired power plants dotted across the country provide about 37 percent of the US electricity supply, ahead of natural gas (30 percent) and nuclear reactors (19 percent).

While the extent of the measures have yet to be disclosed, the main outlines are clear: the administration will set emissions reduction targets for each state and then give them leeway in meeting those caps.

On Sunday, The Wall Street Journal and The New York Times, citing sources briefed on the plan, said the EPA would seek a 30 percent cut in carbon emissions by 2030, as compared with 2005 levels.

States would be given several options for how to achieve the cuts, the reports said.

"There are a lot of very old and inefficient coal power plants," said Kevin Kennedy of the World Resources Institute in Washington.

"This will be another factor in decisions that utilities will need to take into account as they consider what to operate and what to shut down going forward."

- Sensitive issue -

Climate change is a hot-button issue in American politics.

Republicans, who control the House of Representatives, are against any new law touching on it, with some even disputing the existence of global warming. Others cast doubt on whether humans are to blame for the phenomenon.

Stymied on the legislative front, the White House is now poised to act on a regulatory level via the EPA by evoking the Clean Air Act -- an approach criticized by some industry advocates who warn such action could lead to major job losses.

This new initiative -- which also aims to promote renewable energy sources such as solar and wind, as well as energy efficiency -- is part of a larger climate action plan announced by Obama a year ago.

In 2009, the US leader pledged to reduce the country's greenhouse gas emissions by 17 percent by 2020 compared to 2005 levels.

- Cap-and-trade -

Among the paths being pursued is the putting in place of cap-and-trade programs at the state level to encourage energy companies to invest in alternative energy or technology that produces less pollution.

Those whose emissions surpass the fixed ceiling will have to buy additional quotas, while those whose emissions come in below the threshold can sell their leftover capacity.

A regional market of this type already exists in the US Northeast, taking in a dozen states from Maine to Maryland. A similar initiative has been launched in California.

Obama, who made the battle against climate change a core promise of this 2008 election campaign, tried but failed to implement this quota system on a federal level due to opposition from lawmakers.

In his weekly address on Saturday, Obama defended his regulatory approach, stressing that climate change was "no longer a distant threat" but a reality.

"We limit the amount of toxic chemicals like mercury, sulfur, and arsenic that power plants put in our air and water. But they can dump unlimited amounts of carbon pollution into the air," he said.

"It's not smart, it's not safe, and it doesn't make sense."

- 'Killing American jobs' -

Alden Meyer of the Union of Concerned Scientists said the setting of standards for coal-fired power plants is the "centerpiece" of Obama's climate action plan and will be closely eyed abroad ahead of an international climate conference to be held in Paris in late 2015.

"Other countries will be watching quite closely and they will also be waiting to see what the political reaction is: Will it survive the attacks in courts or in Congress to try to block it? Will it be continued by the next president?" Meyer said.

Republican House Speaker John Boehner, when asked about the pending administration announcement, didn't hide his displeasure.

"I'm not qualified to debate the science over climate change. But I am astute enough to understand that every proposal that has come out of this administration to deal with climate change involves hurting our economy and killing American jobs," he said.

"That can't be the prescription for dealing with changes in our climate."