"We recognize that human activity, including the burning of fossil fuels, is contributing to increased concentrations of greenhouse gases in the atmosphere that can lead to adverse changes in global climate," ConocoPhillips Chief Executive Jim Mulva said in a statement.

ConocoPhillips is the No. 3 US oil company and AIG is the world's largest insurer. The US Climate Action Partnership is a group of businesses and environmentalists launched in January to pressure the US government to move quickly to reduce greenhouse gases.

The announcements come a day after a United Nations draft report said global temperatures were on track to exceed a rise of 3.6 degrees Fahrenheit (2 Celsius) over pre-industrial times, and governments have little time left to avert damaging temperature increases.

Last week, the US Supreme Court ruled that the US Environmental Protection Agency had the power to regulate greenhouse gas emissions, rejecting arguments that the body did not have the authority to limit pollution from new cars and trucks.

CRITICIZED INDUSTRIES

The US oil and insurance industries have been widely criticized by environmentalists as being slow to act against climate change.

ConocoPhillips' move made it the second major oil producer behind Britain's BP Plc to join the group and moved it away from US-based oil majors Exxon Mobil Corp. and Chevron, which both have stopped short of endorsing mandatory rules on emissions in the United States.

Both Exxon and Chevron said they had no plans to join the US Climate Action Partnership, but planned to continue their involvement in discussions on proposed legislation.

Oil company operations in Europe and some other parts of the globe are subject to carbon dioxide emission rules that states have enacted to comply with the Kyoto Protocol framework designed to roll back emissions.

ConocoPhillips said any laws requiring emission cuts should be linked to international programs and structured to avoid increasing the volatility of energy prices.

It said it was building the potential long-term cost of carbon into its capital spending plans for major projects around the world and was working to increase energy efficiency at its facilities, including a 10 percent improvement at its US refineries by 2012.

The company is also developing targets to reduce its own greenhouse gas emissions.

CLIMATE HEDGE

Insurers swallowed about US$80 billion in claims from hurricanes wreaking havoc along the Gulf Coast region in 2005, and have been coming under pressure to find ways to prepare for increased catastrophic weather.

But US insurance companies have been conspicuously quiet on the subject of global warming, critics have suggested, in an attempt to placate the businesses they serve.

"As the first insurer to join USCAP, AIG can help shape a broad-based cap-and-trade legislative proposal, bringing to this critical endeavor a unique perspective on the business opportunities and risks that climate change poses for our industry," Chief Executive Martin Sullivan said in a statement.

AIG said the partnership's proposal of a market-driven cap and trade program was one factor that attracted the insurer.

The insurer has been weighing the possibility of adding carbon dioxide emissions credit to the Dow Jones-AIG Commodity Index, providing a platform for investors to trade credits designed to offset greenhouse emissions.