Sept. 18 (Bloomberg) -- The International Energy Agency, an adviser to oil and gas consuming nations, said U.S. natural-gas prices will average $10 a million British thermal units this year.

``A number of factors, including higher oil prices, weather conditions and supply and demand imbalance, all played a role during the price increases in the past 18 months,'' the IEA said in its Natural Gas Market Review 2008. U.S. natural-gas prices have fallen to about $8 a million Btus from more than $13 a million Btus in July.

Spare liquefied natural gas cargoes cost more than $15 a million Btus in the Pacific, the IEA added.

``Flexible liquefied natural gas (spot and short-term) played a greater role in inter-regional market balancing in 2007 and into 2008,'' said Nobuo Tanaka, executive director of the IEA, in an e-mailed statement.

IEA countries will import more expensive gas from further afield, the report said. The agency sees insufficient investment ``particularly in the years beyond 2010.'' LNG production project delays threaten security of supply, it added.

To contact the reporter on this story: Ben Farey in London at bfarey@bloomberg.net

Last Updated: September 18, 2008 06:18 EDT