WASHINGTON (AP) _ A U.S. government-sponsored report backs California's role in setting tough standards on smog-forming emissions from cars, trucks and construction vehicles. The state's approach is scientifically valid and necessary in the quest for cleaner air, researchers said Thursday.

Acknowledging substantial progress in reducing emissions, the National Academies' National Research Council also said more needs to be done to meet federal air-quality standards in many parts of the United States.

California began regulating pollution before the federal government did. Under the Clean Air Act, the state has the power to set its own vehicle pollution standards. Among the other states that have put in place California's rules are Connecticut, Maine, Massachusetts, New Jersey, New York, Oregon, Rhode Island, Vermont and Washington.

``California has used its authority as Congress envisioned: to implement more aggressive measures than the rest of the country and to serve as a laboratory for technological innovation,'' the report said.

The auto industry and environmentalists, which have fought over California's strict requirements, kept close watch on the review, which the U.S. Congress ordered in 2003.

Automakers have said the state's rules add considerable cost for consumers and that the federal standards for nitrogen oxide and hydrocarbons are stringent. Environmentalists worried that legislators might use the study to restrict states' abilities to follow California regulations.

``State initiatives were the kick-in-the-pants automakers needed to produce the cleanest cars on the market today,'' said Michelle Robinson, Washington director of the Clean Vehicles Program for the Union of Concerned Scientists.

The report does not recommend giving the Environmental Protection Agency the power to stop states from following California's standards. Instead, it encourages the agency to give states more guidance on the benefits of the federal rules.

Automakers noted that the study recognized that vehicles have become cleaner since the 1970s. Charles Territo, a spokesman for the Alliance of Automobile Manufacturers, said it also recommended that the EPA ``become more involved in helping states understand the complexities of the regulations and the benefit of the regulations.''

David Allen, chairman of the council committee that produced the report, said manufacturers described the costs associated with complying with both California and federal standards. But it was difficult for the committee to quantify those costs, said Allen, a professor of chemical engineering professor at the University of Texas at Austin.

When the California Air Resources Board revises or sets an emission standard, it must seek a waiver from the EPA. The study recommended that the agency speed up waiver requests it considers routine and put a time limit on more contentious decisions.

The committee recommended that EPA reviews last no longer than two years.

While the study was underway, California air regulators in September 2004 approved the world's most stringent rules to reduce greenhouse gas emissions. The auto industry has challenged those standards in court; the report did not address the subject.