BEIJING (AFP) - China has begun building a 314m high dam which will be among the world's tallest, officials said, as the country massively expands hydropower.

The Shuangjiankou dam on a tributary of China's mighty Yangtze river will be completed in 2022, the environmental ministry said on its website Tuesday.

The facility, costing 36 billion yuan (S$7.9 billion), will be higher than the world's current tallest dam, the 305m Jinping-1, also in China.

China has been expanding its hydropower electricity generating capacity as it seeks to reach a goal of obtaining 20 per cent of its power from non-fossil sources by 2030.

Beijing says reaching the target will help reduce growth in its carbon emissions responsible for climate change, the world's largest.

China is home to the world's largest dam, the Three Gorges, as well as some 85,000 other hydropower facilities.

Millions of people have been forced to relocate because of dam building in the country in recent decades, often with little compensation.

Environmentalists say that the dams have caused large declines in fish stocks and other aquatic life.

"The true costs of hydropower in China are consistently being underestimated or ignored, including the environmental and social costs," the US-based NGO International Rivers said in November.