US emissions increased to 6.94 billion tonnes carbon dioxide equivalent in 2003, up from 6.89 billion tonnes in 2002.

This represents roughly 24 percent of total world emissions in 2003, according to a report produced by the Energy Information Administration, the statistical arm of the Energy Department.

Greenhouse gases -- primarily carbon dioxide -- are suspected of contributing to a gradual warming climate, a change that some believe will threaten weather patterns, impair agricultural crops, and lead to the melting of ice caps.

The US has faced growing international pressure to reduce its greenhouse gas emissions after it opted out of ratifying the Kyoto climate change protocol.

The protocol, set to take effect February 16, 2005, imposes caps on the emissions levels of those industrialized nations that have ratified the treaty in an effort to reduce emissions to 1990 levels by 2015.

US greenhouse gas emissions have climbed a total of 13.4 percent above the 1990 level, or roughly one percent a year between 1990 and 2003, the report said.

The bulk of emissions, 83 percent, consisted of carbon dioxide produced from the combustion of coal, petroleum, and natural gas.

"US emissions trends are driven largely by trends in fossil energy consumption," the report said. Emissions of methane-stemming from landfills, coal mines, animal waste, and natural gas systems-increased by 0.5 percent in 2003 compared to 2002, but are still 15 percent below the 1990 level.

Washington has argued that when measuring emission as a percentage of economic output, the United States is more efficient than many other nations.

"The 2003 increase is well below the rate of economic growth of 3.0 percent and below the average annual growth rate of 1.0 percent in greenhouse gas emissions since 1990," the Energy Department said.

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