FILE PHOTO: Bottles of polluted water from Xiaohaotu county, Yulin of Shaanxi province, are displayed in a mini van at a roaming exhibition art project by Chinese performance artist "Nut Brother", in Beijing 798 art area, China July 12, 2018. REUTERS/Jason Lee

BEIJING (Reuters) - China’s surface water quality improved in 2018, with more samples taken from river and lakes reaching standards fit for human use, the country’s environment ministry said on Monday.

Beijing has extended its campaign to tackle pollution from air to water, attempting to clean up the black and stinky streams flowing through Chinese cities and improve the water quality of its natural reserves.

But while water quality in most of China’s major waterways including the Yellow, Huai, Yangtze and Pearl rivers improved in 2018, those in northeastern China such as the Liao and Songhua were found to be even more polluted than in 2017.

China has vowed to take action to “significantly reduce” the volume of industrial wastewater flowing into the Bohai Sea in northern part of the country.

Among the 1,940 samples from across China last year, 71 percent were considered grade III or better, meaning they were suitable for drinking and fishing, the Ministry of Ecology and Environment (MEE) said, up 3.1 percentage points on 2017.

Meanwhile the amount of “below grade V” water that cannot be used in either agriculture or industry, fell by 1.6 percentage points to 6.7 percent in 2018.

Phosphorus and ammonium nitrate, which mainly come from industrial waste water, pesticides and organic fertilisers, were identified as the major pollutants of water pollution, the MEE said in a statement.

Chemical oxygen demand, a measure used to determine organic content in water, was also found to exceed the national level in the “below grade V” rivers.