WASHINGTON (AP) - The Environmental Protection Agency has concluded greenhouse gases are endangering Americans' health and must be regulated, signalling that the Obama administration is prepared to contain global warming without congressional action if necessary.

The finding is timed to boost the administration's arguments at an international climate conference - beginning this week - that the United States is aggressively taking actions to combat global warming, even though Congress has yet to act on climate legislation.

Under a Supreme Court ruling, the so-called endangerment finding is needed before the EPA can regulate carbon dioxide and five other greenhouse gases released from power plants, factories and automobiles under the federal Clean Air Act.

The EPA signalled last April that it was inclined to view heat-trapping pollution as a threat to public health and welfare and began to take public comments under a formal rulemaking. The action marked a reversal from the Bush administration, which had declined to aggressively pursue the issue.

Business groups have strongly argued against tackling global warming through the regulatory process of the Clean Air Act. Any such regulations are likely to spawn lawsuits and lengthy legal fights.

The EPA and the White House have said regulations on greenhouse gases will not be imminent even after an endangerment finding, saying that the administration would prefer that Congress act to limit such pollution through an economy-wide cap on carbon dioxide and other greenhouse gases.

Nevertheless, the EPA has begun the early stages of developing permit requirements on carbon dioxide pollution from large emitters such as power plants. The administration also has said it will require automobile fuel economy to increase to a fleet average of 35 miles per gallon by 2016, another push to reduce carbon dioxide emissions.

While the House has approved climate legislation that would cut emissions by 17 per cent by 2020 and about 80 per cent by mid-century, the Senate has yet to take up the measure amid strong Republican opposition and reluctance by some centrist Democrats.