The move could allow the US Environmental Protection Agency (EPA) to order cuts in emissions without the approval of Congress.

EPA administrator Lisa Jackson said the agency was now "authorised and obligated to make reasonable efforts" to cut greenhouse gases.

The news came as global climate talks got under way in Copenhagen aimed at forging a deal on major emissions cuts.

Ms Jackson said that the scientific evidence surrounding climate change clearly showed that greenhouse gases "threaten the public health and welfare of the American people".

She said the EPA's so-called endangerment finding would "cement 2009's place in history as the year when the United States government began addressing the challenge of greenhouse-gas pollution".

Reports suggest the announcement, which had been in the pipeline for months, was timed to add weight to US President Barack Obama's position at the climate change talks in Copenhagen, allowing him to argue that the US is taking action to combat global warming.

However, the president's preferred way of addressing greenhouse gas emissions is still through the legislative process, says the BBC's environment correspondent Richard Black.

The president is said to favour a cap-and-trade bill backed by Senators Barbara Boxer and John Kerry which is currently making its way through the US Senate.

Our correspondent says a bill would give political backing to curbs on emissions.

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A secondary motive, he says, is that prevailing economic theory predicts cap-and-trade to be a more efficient way of reducing emissions. That is because it uses a market mechanism, whereas acting through the EPA could mean mandating cuts.

Using cap-and-trade legislation as well as EPA mechanisms could in principle reduce US emissions more than the target it has put before the UN climate summit, our correspondent says.

And campaigners will be urging Mr Obama to pledge bigger cuts and use every power at his disposal.

However, while the House of Representatives already passed a cap-and-trade bill, progress through the Senate is not likely before March at the earliest.

'Health hazard'

In April this year, the EPA decided that carbon dioxide and five other greenhouse gases could endanger human health and well-being.

The decision was then sent out to public consultation, with people given 60 days - until 23 June - to respond.

An EPA spokeswoman told the BBC that the agency had received more than 300,000 comments and had been working on its response since.

Under a Supreme Court ruling, the EPA's "endangerment finding" was needed to allow the agency to regulate carbon dioxide and five other greenhouse gases released by vehicles, power plants and factories under the federal Clean Air Act.