Some Chinese regions near key waterways the Yangtze and Yellow rivers are "seriously lagging" state water quality standards despite big improvements overall this year, the environment ministry said.
China divides its water resources into six grades, with the top three considered safe for human use, and the lowest, "below grade V", unfit even for industrial or irrigation purposes.
Some 75.4% of China's water was judged to be in the top three grades in the first 10 months of this year, up 2.3 percentage points from a year earlier, the Ministry of Ecology and Environment said late on Wednesday.
Just 3.2% was found to be "below grade V", down 1.9 percentage points on the year.
However, "some regions were seriously lagging in their water environment targets", the ministry said.
Parts of the Yangtze river and Bohai Bay, near the industrial region of Tianjin, needed to work harder to eliminate water "below grade V", it added.
The Yellow river also required "urgent" pollution treatment work on its middle and lower reaches.
China's per capita water resources are around a quarter of the global average, and with demand still rising, it is trying to remediate contaminated water sources and improve efficiency.
It is in the middle of a wide-reaching programme to clean up the Yangtze, its biggest river, and put an end to major development along its banks.