Announcing the initiative, Secretary of State Hillary Clinton acknowledged the plan does not address carbon dioxide emissions, the primary cause of global warming that many governments are reluctant to cap.

The six-country coalition, which also includes Bangladesh, Canada, Mexico, Sweden and Ghana, will tackle "short-lived" climate pollutants, highly potent greenhouse gases that contribute to one-third of global warming, according to the State Department.

Unlike carbon dioxide emissions, which remain in the atmosphere for a century, "short-lived" pollutants stay in the atmosphere for a few days to years.

Curbing black carbon, methane and HFCs, gases used in refrigerants and aerosols, can help the world achieve the U.N. goal pledged by nearly 200 countries of limiting the rise in global temperatures to 3.8 degrees Fahrenheit (2 degrees Celsius) by 2050, Clinton said.

The United States is the world's second biggest emitter of greenhouse gases blamed for global warming, behind China. Several attempts to pass legislation to limit such emissions have failed in the U.S. Congress.

The new initiative will combine existing programs that address the pollutants, such as a global clean cooking stove initiative that tackles black carbon, or soot; and a global partnership led by the U.S. Environmental Protection Agency to tackle emissions from methane in coal mines, agriculture and natural gas and oil systems.

The United States will contribute $12 million and Canada $3 million to the program's start-up funding.

"We know of course that this effort is not the answer to the climate crisis. There is no way to effectively address climate change without reducing carbon dioxide, the most dangerous, prevalent and persistent greenhouse gas," Clinton said.

Environmental groups applauded the initiative but warned that the coalition should not distract from the wider effort to tackle carbon emissions.

"Going after black carbon, methane and other short-lived climate forcers is no substitute for a strong, sustained effort to significantly reduce emissions of carbon dioxide, the main driver of climate change," said Eileen Claussen, president of the Center for Climate and Energy Solutions.

WWF said the U.S. and Canada should not shift the focus on other countries to address climate change, especially because both countries have failed to enact global warming bills.

"While support for poorer countries is important, their primary responsibility should be to cut their own emissions and address the global challenges posed by climate change," said Samantha Smith, leader of the WWF Climate and Energy Initiative.

The six countries launching the initiative will hold their first meeting in Stockholm in April.