The chief economist of the International Energy Agency has told a U.S. newspaper that China is set to overtake the United States this year as the world's biggest source of greenhouse gasses.

Earlier estimates had suggested China would surpass the U.S. in gas emissions by 2010.

In an interview published in The Wall Street Journal Tuesday, the IEA's Faith Birol said Beijing's refusal to limit greenhouse gases will allow China to increase its emissions nearly unchecked.

She said this could effectively cancel out other countries' efforts to cut their own emissions. Birol said that could weaken efforts to design a global greenhouse gas treaty to replace the Kyoto Protocol.

The Kyoto Protocol requires 35 industrial nations to cut emissions by 2012 but excludes developing nations such as China and India from specific targets.

Seventy percent of China's energy output comes from burning coal - a heavily polluting energy source.

Chinese officials say their country is a developing nation and lacks the resources to cut emissions and switch to cleaner forms of energy such as water, wind and solar power. They say developed nations should take the lead in cutting greenhouse gas emissions.

Some information for this report was provided by AP.