SHANGHAI--China aims to double the amount of water it transfers from the flood-prone south to arid northern regions, officials said on Thursday, as the government prepares to launch the second phase of its controversial cross-country water diversion scheme.

The South-North Water Diversion Project was first proposed in 1952 to ease flooding in the south and drought in the north, but critics say its costs are too high and the diversion of polluted water to other regions could contaminate other lakes and rivers.

The first phase of the project, completed five years ago, linked the Yangtze and Yellow rivers through two main routes in eastern and central China, with another, more challenging route in the far west still to come.

Preliminary work is now underway on the second phase, which will raise annual delivery capacity from 8.77 billion cubic meters to 16.5 billion cubic meters, said Shi Chunxian, head of the planning office of the Ministry of Water Resources.

Shi told reporters that phase II would supply the provinces of Anhui and Shandong as well as regions around Beijing, adding that China will make full use of existing infrastructure to minimize the expansion's environmental impact.

The project has so far delivered a total of almost 30 billion cubic meters of water to the north in five years, supplying 120 million people, vice-water minister Zhang Youguang told the briefing.

However, though 345,000 have been relocated to make way for the project, critics say it doesn't address China's problems, including excess water consumption from industrial consumers as well as leaky urban pipeline systems.

Many Chinese cities have come to rely increasingly on elaborate water diversion schemes, with China still prioritizing grand engineering feats over tackling pollution or improving conservation and efficiency.

"Replacing underground pipes is not nearly as sexy as building the biggest inter-basin diversion on the planet," said Darrin Magee, a professor at the Hobart and William Smith Colleges who specializes in China's water issues.

China's per capita water resources are around a quarter of the global average, and Premier Li Keqiang said last month that new channels were needed to deliver more water north and tackle growing supply concerns.

Construction has also begun on an "emergency" extension that will divert another 490 million cubic meters of water a year to Beijing and surrounding regions. Beijing already relies on the project for 70 percent of supplies and that could rise to 95 percent.

Despite calls from President Xi Jinping to put an end to large-scale development on the Yangtze river, big diversion schemes look set to continue.

"I think that greening the Yangtze, while surely a priority, doesn't quite have the urgency from a social stability point of view that making sure there's water available in Beijing and Tianjin," said Magee.