The U.S. Supreme Court is hearing a case Wednesday intended to force the federal government's environmental agency to regulate some greenhouse-gas emissions.

A group of applicants including 12 states, 13 environmental organizations and three cities wants the U.S. Environmental Protection Agency to regulate carbon dioxide tailpipe emissions from new vehicles. Carbon dioxide is believed to contribute to global warming.

The applicants say that the EPA should regulate the emissions under the Clean Air Act because they are air pollutants which could harm public health.

The EPA said it cannot regulate global warming under the act, and therefore carbon dioxide, as a greenhouses gas, is not an air pollutant.

The agency also said it would not regulate such gases, even if it could, because there is uncertainty about global warming.

The government position is supported by car-industry groups and nine states where vehicles are made.

The applicants took their case to the top court after the EPA refused an environmental group's request to regulate emissions. A lower court in Washington, D.C., backed the EPA.

The Supreme Court is expected to rule by July 2007.

The court is also hearing a second, related case regarding an EPA decision that it lacked the authority to regulate greenhouse-gas emissions from power plants.

With files from the Associated Press