TOKYO (Reuters) - Toyota Motor plans to sell a new version of its Prius petrol-electric hybrid from the end of 2015 in Japan and in other regions thereafter, at a time when tumbling gasoline prices are sapping demand for environment-friendlier cars.

The new Prius will run on smaller, lighter motors and batteries than its predecessor, and be at least 10% more fuel-efficient, Toyota said, without stating mileage. The previous model can run at 51 miles a gallon.

Thermal efficiency - or ratio of work performed to energy consumed - will be 40 percent, up from 38.5 percent, the company also said.

The car, unveiled in Las Vegas late on Tuesday, is the first model built under the Toyota New Global Architecture (TNGA) strategy, a new approach to development to improve its cars' fuel efficiency, styling and other features, partly by using more common components.

The launch comes at a challenging time for Toyota. U.S. petrol prices are at their lowest for this time of year since 2004, pushing consumers to trade in hybrids and electric vehicles in favor of sports utility vehicles.

August Prius sales in the U.S. fell 24 percent from the same month a year earlier to 17,757 cars. Last year, U.S. Prius sales totaled 207,372 vehicles.

Other automakers are adjusting product launches as cheap petrol muddies the sales outlook for green cars.

General Motors will limit the sale of its next-generation Chevrolet Volt plug-in hybrid - slated to launch in coming months - to California and other states where hybrid and electric cars sell well, a Chevrolet spokesman said.

(Reporting by Minami Funakoshi; Additional reporting by Joe White; Editing by Chang-Ran Kim and Christopher Cushing)