US greenhouse gas emissions, which include carbon dioxide, methane and nitrous oxide, have risen 15.8 percent from 1990 to 2004, according to the EPA.

President George W Bush says US greenhouse gas intensity, or the amount of greenhouse gas emission for every dollar of economic output, is falling. He prefers voluntary methods of cutting emissions.

The United Nations' Kyoto Protocol on global warming signed by 156 countries requires developed nations to reduce greenhouse gas emissions by 5.2 percent of their 1990 levels from 2008 to 2012.

President George W Bush pulled out of the Kyoto pact in 2001, saying it would harm the economy.

So far, adherence to the Kyoto goals has been patchy. The UN climate office said earlier this month that Kyoto nations were on track to cut emissions by 3.5 percent compared with 1990 levels by 2012 but that the goal of 5.2 percent could be reached by introducing extra measures.

The EPA did not indicate how US emissions would fare in the future, but climate researchers say voluntary cuts won't trim overall greenhouse output in the United States.

"Voluntary programs that are aimed at reducing emissions...are offset by the growth because there are no requirements overall," said Vicki Arroyo, director of policy at analysis at the Pew Center on Global Climate Change in Virginia.

At a UN conference in December the United States agreed to participate in talks about extending Kyoto Protocol beyond its first phase, but only after the text was watered down to say the discussions would not lead to mandatory emissions caps.