Taking the top spot would focus pressure on China to do more to brake emissions as part of world talks on extending the United Nations' Kyoto Protocol on global warming beyond 2012.

Thirty five developed nations have agreed to cut emissions under Kyoto and they want others -- especially the United States and China -- to do more.

"It looks likely to me that China will pass the United States this year," said Gregg Marland, a senior staff scientist at the US Carbon Dioxide Information Analysis Center (CDIAC), which supplies data to governments, researchers and non-governmental organisations worldwide. "There's a very high likelihood they'll pass them in 2007."

Carbon dioxide (CO2) is produced by burning fossil fuels like coal, oil and gas for heat, power and transport. Most scientists say it is a key contributor to global warming.

Marland used fossil fuel consumption data from oil company BP to calculate China's CO2 emissions in 2005 at 5.3 billion tonnes, versus 5.9 billion for the US, with respective growth in 2005 of 10.5 percent and less than 0.1 percent.

In 2006 Chinese fuel consumption rose 9.3 percent to the equivalent of 2.4 billion tonnes of coal that year, the deputy head of the office that advises China on energy policy, Xu Dingming, said on Thursday.

This was faster than BP's estimate of a 9 percent rise in China's oil, gas and coal consumption in 2005, to 1.45 billion tonnes of oil equivalent.

The International Energy Agency (IEA), which advises 26 rich nations, had already said last November that on current trends China would overtake the United States as the world's biggest carbon emitter before 2010.

China's Office of the National Coordination Committee on Climate Change said it could not comment on either forecast as it did not have a reliable estimate of the country's emissions.

"These figures are very complicated -- we don't have an estimate of CO2 for such a recent date," said an official who declined to be named. "We have just set in motion our national reporting plan... but it will not be done for two or three years."

US CITIZENS STILL TOP EMITTERS

UN data for 2003 put the US top with 23 percent of world carbon dioxide emissions and China second on 16.5 percent. But US individuals were far bigger emitters, at 20 tonnes per capita against China's 3.2 tonnes and a world average of 3.7.

China argues that wealthy nations are responsible for most of the greenhouse gases already in the atmosphere and should lead the way in cutting emissions.

And much of the growth in China's emissions is to produce goods consumed in the West, raising ethical questions over who bears responsibility for those emissions.

Higher economic growth and fuel use translates into higher emissions, particularly in China, which gets around 70 percent of its energy from coal, the highest carbon-emitting fuel.

CDIAC's 2004 emissions estimates, based on BP data, closely matched the IEA's estimates for the same year -- reached using its own energy data and UN emissions calculation methods, strengthening the reliability of the BP data, Marland said.

He estimated a plus or minus 15 to 20 percent error in the Chinese data versus a possible 5 percent US error margin.

China's rapid growth in carbon emissions is threatening to outweigh efforts by the European Union and others to tackle climate change -- EU leaders said earlier this month they would cut the bloc's greenhouse gases by at least a fifth by 2020.

But China between now and 2015 will build power generating capacity equal to the entire existing capacity in the whole of the European Union, the IEA estimates.

China's growth has been fuelled largely by burning coal, and it is still building new power plants at an unprecedented rate. Last year alone it added around 100 gigawatts of new generators, approaching France's entire capacity, most of them coal-burning.