US emissions from the burning of oil, natural gas and coal typically increase by one percent a year, according to the average determined by the Energy Information Administration (EIA) for the period of 1990-2006.

The decline last year was the sharpest since 1990.

The United States emitted 5.88 billion metric tonnes of carbon-dioxide produced from fossil fuels in 2006, down from 5.95 billion tonnes in 2005, the EIA said in preliminary estimates.

Among the factors reducing the greenhouse emissions blamed as contributing to global warming was an unusually warm winter and a cooler summer that reduced the demand for heating and air conditioning.

Higher energy prices for natural gas, motor fuel and electricity reduced energy demand and in addition a less carbon-intensive fuel mix -- more natural gas and non-carbon fuels -- was used, the EIA said.

Between 1990 and 2006, energy-related US carbon-dioxide emissions increased by 17.9 percent.

That type of emission accounts for more than 80 percent of US greenhouse-gas emissions in the country.