The American federal Government is supporting legal action at the Supreme Court contesting a Californian clean air agency's moves to reduce pollution from fleet vehicles such as buses and taxis.

The US Department of Justice made a filing late on Friday, calling on the court to overturn the South Coast Air Quality Management District's clean fleet rules for the greater Los Angeles metropolitan region.

The agency's laws, passed in 2000 and 2001, oblige fleet operators to buy clean-fuelled models when they replace or add vehicles to their fleets, leading to the replacement of thousands of diesel models.

Hundreds of natural gas-burning and other 'green' vehicles have replaced defunct machines since the regulations came into place, at considerable cost.

Industry groups the Western Petroleum Association and the Engine manufacturers Association sued the AQMD in the District and Appeals courts, but having failed both times, are to take their objections to the Supreme Court.

The Justice Department's friend-of-the-court brief argues that the federal Clean Air Act prevents states and local jurisdictions from establishing their own emissions standards for new vehicles.

The AQMD contests the view that its rules represent a new set of standards, arguing simply that it asks operators to choose replacements from among the cleanest products available, with exceptions granted when such alternatives cannot be found.