WASHINGTON, Feb 3 - The United States expressed disappointment Wednesday over a decision by Japan to limited the number of American-made cars eligible for incentives under Tokyo's "cash for clunkers" auto stimulus program.

US Trade Representative Ron Kirk reacted to Japan's use of a fuel economy formula based on urban driving to be eligible for the incentives, which limits the number of US models that qualify.

"We are disappointed with Japan’s announcement today to limit the number of US autos models that qualify for its ‘cash-for-clunkers’ program." Kirk said in a statement.

"This is particularly unfortunate in light of its recent announcement to open opportunities for US autos to qualify for its program, which was a welcome step. We will continue to urge Japan to implement its program in a manner that is transparent and as inclusive of US autos as possible. We will also continue to monitor developments closely to ensure US autos indeed benefit from Japan's 'cash-for-clunkers' program."

US officials said Japan had decided to use the fuel economy ratings based on "city" driving instead of the "combined city/highway" mileage ratings as the basis for determining eligibility, thus limiting the number of US models that qualify for the program.

The comments marked the latest in a spat over the Japanese program, which originally had excluded US-made cars but was modified last month following protests from Washington.

In 2008, Japan exported more than two million cars and light trucks to the United States, but imported just 12,181 cars and light trucks from the United States, congressional reports said.