Though water supplies have resumed, the melting ice and snow could cause pollution problems when chemicals thaw, the UN Environment Programme said.

"Great care must be taken in the spring when the ice thaws out," the UN body said in a report released on Thursday, which followed a trip to northeast China.

"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," it said.

But the official China Daily on Friday downplayed the chances of a new toxic slick, quoting officials as saying that even if some chemicals were released, the level would not be high enough to force water supplies to be cut off again.

The UNEP said it was ready to help China deal with the aftermath of the accident, and to help stop such disasters from happening in the future.

"An analysis of the internal risk management practices of industry should be undertaken through a random sample of industries," the report added, saying both China and Russia should provide access to independent sampling.

The report also criticised China for its slow initial response to dealing with the crisis in the Songhua River.

"Available information showed unverified media information and initial lack of proper communication from government authorities had caused some panic," it said.

China has vowed to get tough on pollution and has ordered local authorities to inform the central government directly there are any environmental crises.

China's top environmental minister resigned after the accident and a vice mayor in charge of evacuating the city where the explosion occurred was said to have hanged himself.

But the United Nations said its mission was not allowed to visit the accident site, could not take its own samples and only made it to the area four weeks after the disaster.

"By this time, it was hard to find out when certain measures had started and what response time various actors required," the report said.