HONG KONG (AFP) – China could overtake the United States as the world's largest economy if it maintains annual growth of eight percent over the next 20 years, the World Bank's chief economist said Wednesday.

China, which last year overtook Japan as the number-two economy, has already set a growth target of eight percent for 2011, and is aiming for seven percent a year from 2011 to 2015.

"China may become the largest economy in the world by 2030," Justin Lin told an economic forum on China development in Hong Kong, saying its economic size may then be twice as large as the US, measured by purchasing power parity.

He said China has been the world's fastest growing nation over the past two decades and grew at a "miracle" annual growth rate of 10.4 percent between 1990-2010, a sharp contrast to the performance of other transitional economies.

The economist however warned expansion may be hampered by a weak global recovery from the financial crisis and urged the country to deal with domestic challenges such as addressing its increasing income inequality.

He also said global warming may pose a "real challenge" to China's long-term sustainability, but urged China -- the world's biggest polluter -- to seize the opportunity and instead turn itself into a new leader in green technology.

"It's necessary for China to continue its growth. These are challenges for China but they are also opportunities for China," said Lin, who assumed the World Bank post since 2008.

China has unveiled ambitious programmes to develop clean energy and shut down factories that fail to meet emissions targets, aiming to reduce its carbon intensity by 40 to 45 percent by 2020 based on 2005 levels.

China's economy grew 10.3 percent in 2010, marking the fastest annual pace since the onset of the global crisis.

Unlike other countries struggling to spur growth, Beijing has been trying to slow its economy and stem a flood of liquidity that is fanning inflation and driving up property prices.