China and Russia Friday started a new round of joint monitoring over the water quality of four cross-border rivers and one cross-border lake.

Twelve Chinese environmental experts and five Russian experts took water samples from a section of the ice-covered Heilongjiang River, or Amur River, in Heihe City, northeast China's Heilongjiang Province.

The samples will be tested for 40 types of pollutants, said Sheng Shuzhong, deputy director of the city environmental protection bureau in Heihe.

The experts are also scheduled to take water samples in May, June and August this year.

The five-year joint monitoring operation also covers the Wusuli River, Ergune River, Suifen River and Xingkai Lake.

The two countries launched a four-year joint water-quality monitoring of the boundary waters in 2007 following a chemical spill into the Songhua River, a major tributary of the Heilongjiang River, in northeast China in late 2005.

"The joint monitoring showed the water quality in the cross-border waters has improved markedly," said Sun Qingmin, a deputy inspector of the Heilongjiang Provincial Bureau of Environmental Protection.