WASHINGTON — General Motors yesterday became the first automaker to announce plans to build a plug-in hybrid vehicle, improving the chances that cars of the future would go much farther on a gallon of gas.

Plug-in gas-electric hybrids would significantly expand gas mileage through the use of advanced batteries that would provide greater range under electric power than current models. The cars would rely much less on conventional engines, reducing gasoline consumption and pollution.

GM Chairman and Chief Executive Richard Wagoner made the announcement at the Los Angeles Auto Show but, citing technical obstacles, didn't say when the new vehicle would go on sale. Wagoner said it will take several years for GM to create a plug-in hybrid that would meet performance standards. He acknowledged that affordable battery technology doesn't exist yet. Toyota and Ford also are studying the technology.

GM's initiative reflects a belated attempt to win an image as a "green" car company and to catch up with the alternative-fuel efforts of Toyota, the world's leader in hybrid vehicles with its popular gas-electric Prius. Toyota is far ahead of U.S. automakers in creating software, electrical systems and other high-tech parts that the company says could form the basis of plug-in technology. "We want it as much as anybody else but there are limits right now in terms of technology. It's not lack of desire. It's lack of science," James Press, Toyota's U.S. chief executive, said yesterday.

Toyota and Honda have benefited from their early push into hybrids, while GM was promoting the Hummers and other large sport utility vehicles that powered its sales in the 1990s.

After largely ignoring electric alternatives, U.S. officials are now putting their weight behind plug-ins. Members of Congress have flocked to ride-and-drive events featuring plug-in prototypes.

President Bush mentioned the technology in this year's State of the Union address. He also visited a battery plant in the Midwest. Next year, administration officials say, Bush plans to find $14 million in the budget to study plug-ins.

Local government leaders also support the technology and electric utility companies, seeing opportunity to sell electricity as a transportation fuel, are enthusiastic.

"These aren't things that are so distant or beyond the horizon that we cannot even touch them," Alexander Karsner, an assistant secretary of energy, said yesterday. There were no bogeymen in the administration or in Detroit fighting the technology, he said: "This isn't a David and Goliath play. This is a transformation of the American automobile industry that is inevitable."

Gas-electric hybrids have both internal combustion engines and electric motors. Depending on driving conditions, the cars switch back and forth between the two. Gas mileage can be more than 40 miles per gallon.

Plug-ins would have larger, more sophisticated batteries that could store significantly more energy. As envisioned, the vehicles could go 20 to 40 miles on electric power alone. Cars would be plugged into garage outlets to recharge.

Roland Hwang, senior policy analyst at the Natural Resources Defense Council, said he wondered whether GM's announcement was "smoke and mirrors" or the real deal.

"The answer to that question is going to determine GM's competitiveness in the marketplace in the future," Hwang said. "We have seen so many times where GM has made announcements and they have not lived up to their word."

GM created an electric car, the EV1, in the 1990s but dropped it. The company's rejection of the technology was the subject of the recent film "Who Killed the Electric Car?"

In Los Angeles, Wagoner said GM will use the Saturn Vue Green Line, the automaker's only hybrid vehicle, as the basis of its plug-in hybrid. He also said some Hummer models will operate on biofuels within three years and that GM plans to deliver fuel-cell-powered SUVs to consumers next fall as test vehicles. In some of his strongest statements to date on energy issues, Wagoner said that since 2001 a variety of events — including extraordinary growth in China and India, conflicts in the Middle East and global climate change — suggest an "increasingly uncertain energy future on a global basis." He said it's "highly unlikely" that oil alone will fulfill all of the world's automotive fuel needs.

U.S. government support was evident in April when the Department of Energy hosted a conference to discuss the many technical challenges of plug-in hybrids. The conference was attended by auto companies, representatives from public utilities, oil companies, environmental groups, national laboratory scientists and officials from federal agencies including the Environmental Protection Agency, the Department of Transportation and the Department of Defense.

Don Hillebrand, director of the Center for Transportation Research at the government's Argonne National Laboratory near Chicago, said the Japanese government is spending more than $100 million to develop a plug-in battery.

Earlier this year, Hillebrand traveled to Tokyo to attend a Japanese industry event on plug-in hybrids. After giving his presentation, Hillebrand was struck by the depth of the interest during a question-and-answer period. He said it appeared to him that participants reviewed his presentation line by line.

"They brought me over to talk, not to learn," Hillebrand said. "They wanted to hear the U.S. position. There's a strong suspicion that the U.S. has a leg up."