The United States has proposed ordering cuts of up to 30% in carbon emissions from power plants in President Barack Obama's most ambitious action yet on climate change.
The Environmental Protection Agency laid out a proposal that would let states choose ways to meet the goal of lowering emissions by an average of 30% from 2005 levels by 2030.
Power plants account for some 40% of US emissions of carbon dioxide, the most common greenhouse gas that contributes to climate change.
Cuts are politically sensitive as coal, among the dirtiest energy sources, remains a major US industry.
The move comes amid mounting signs of climate change. A UN panel of scientists warned in April that polluters needed to act urgently to avoid worst-case scenarios, which could include increased droughts, storms and coastline destruction.
The Obama administration sought to cast the decision as primarily focused on public health. The environmental regulator said that the cuts would prevent up to 6,600 premature deaths and up to 150,000 asthma attacks in children.
"By leveraging cleaner energy sources and cutting energy waste, this plan will clean the air we breathe while helping slow climate change so we can leave a safe and healthy future for our kids," said Gina McCarthy, the agency's administrator.
The courts have given the environmental agency the right to regulate carbon and Mr Obama has increasingly relied on its power, with Congress unlikely to enact binding cuts in greenhouse gas emissions.
A proposal backed by Mr Obama to require binding cuts through a so-called cap-and-trade system died in the Senate in 2010.