The United Nations Environment Agency (unea) has warned that the spring thaw of ice in China's Songhua river may release toxins trapped from an earlier chemical spill.

"Great care must be taken in the spring when the ice thaws out. The frozen pollutants in the ice will become liquid and gas and the denser liquid that may have stayed in the bottom layer upstream may become mobile as the water flow increases,' the unea said in its report.

The chemical spill had occurred in November in a chemical plant in northern China. It had put a temporary halt to the water supply, which has been resumed.

The Chinese authorities feel the water is safe after the cleanup. However, the unea has underlined the need to be cautious.