WASHINGTON (Reuters) - The US Government needs to move more quickly on plans to boost automobile fuel efficiency standards, improve efficiency of power plants and take hard action on heat-trapping greenhouse gases, the International Energy Agency (IEA) said on Friday.

The IEA, energy advisor to 27 industrialised countries, applauded the US Congress for passing a law in December that boosts the fuel efficiency for cars and trucks for the first time since 1975.

However, the IEA pointed out many European nations as well as Japan and China currently have stricter standards in place than the new US Corporate Average Fuel Economy standards for cars and light trucks that won't take full effect before 2020.

"That's not very fast or ambitious enough," IEA executive director Nobuo Tanaka told reporters at a briefing. "If these kinds of efficiency gains can be achieved outside the US, then why not here?"

The new fuel-efficiency standards are the result of a hard-fought deal with Detroit automakers like General Motors Corp, who had warned that faster timetables could put more burdens on struggling US automakers.

A US Energy Department official also present at the briefing said that fuel standards were only one element of a US strategy to reduce gasoline consumption that also includes plug-in hybrids, a switch to ethanol fuel made from switchgrass and wood chips, and hydrogen-powered cars.

"You have to have a broad portfolio that will allow us to still have consumer choice, allow us to reduce our dependence on liquid petroleum, and at the same time over time transform our vehicle fleet," said Karen Harbert, the department's assistant secretary for policy and international affairs.

US drivers could cut gasoline use by 20 percent if they embraced diesel-fueled cars like European drivers have, said Andreas Biermann, an IEA analyst.

Though US consumers likely have negative memories of the last US experiment in the 1980s, "modern diesel has very little to do with the slightly refined tractors that were coming then," Mr. Biermann said.

Also, gasoline-powered cars can gain efficiency advantages by installing engines that automatically shut down at red lights and brakes that recover energy from spinning wheels as they stop, he said.

The US is also falling behind on improving the efficiency of electric power plants, the IEA said.

While other industrialized nations have improved generation efficiency dramatically over the last decade, "the US has stood still, despite the introduction of new, efficient technology," the IEA said.

US utilities should make more use of existing technology that uses ultra-supercritical pulverised coal plants, which could reduce emissions by 20 percent "without requiring the introduction of unproven technology," the IEA said.