The national average price of diesel fuel at the pump dropped 2.1 cents this week, according to the U.S. Department of Energy. It is the 12th straight week the price of diesel, which now stands at $1.173, has fallen, according to the department's Energy Information Administration.

Gasoline prices have plunged more than 40 cents a gallon to $1.15 since the September 11 terrorist attacks, reaching a two-year low, according to the Lundberg Survey of about 8,000 stations nationwide.

Oil prices may fall further if OPEC doesn't form an alliance with rival independent oil-producing countries for global oil output curbs aimed at lifting world crude prices. Cartel ministers lobbied independent producers to push for the full 500,000 barrel-a-day cut that OPEC has wanted as a condition for triggering its own supply curbs, OPEC sources told the media.

On the other hand, prices may rise if military action in the Middle East spreads to any oil-producing nations, analyst Trilby Lundberg said.

Crude oil for January delivery fell as much as 76 cents to $18.28 a barrel on the New York Mercantile Exchange. Prices have plunged by a third since September 11, which deepened the recession and curbed U.S. energy demand. In London, Brent crude oil for January settlement fell as much as 87 cents, or 4.6%, to $18.16 a barrel on the International Petroleum Exchange.