The number of vehicles on China's roads will more than double to at least 200 million by 2020, a top official was quoted Monday as saying, further straining the nation's environment and energy supply.

China must make it a top priority to develop fuel-efficient and alternative energy cars, the China Securities Journal said, citing Wang Fuchang, vice minister of the Ministry of Industry and Information Technology.

China's auto sales hit 13.64 million units last year, overtaking the United States as the world's top car market, while sales this year are forecast to hit 15 million units.

As of the end of 2009, there were 76.2 million vehicles in the country, according to government figures.

China's auto sales have slowed in recent months, partially due to seasonal factors, but August saw a surprising 55.7 percent year on year jump, boosted by Beijing's new subsidies for energy-saving vehicles.

The surging car use has brought mounting concerns over pollution, soaring energy demand, and traffic gridlock.

A top official with the country's environment minister, Liu Ziquan, was quoted Monday as saying vehicle exhausts had overtaken other major sources as the top cause of air pollution in cities.

Road congestion has also worsened, highlighted by a recurring traffic jam dozens of kilometres (miles) long on a major highway leading into Beijing from the northwest.