The spring thaw could release more toxins into the water from a Chinese chemical explosion that occurred last year, the UN said in a report, adding China should carry out random checks to prevent a recurrence.
The Nairobi-based UN Environment Program (UNEP), which sent a team to China's northeast to investigate damage to the Songhua River and surrounding areas caused by the November spill, said Chinese authorities had reacted too slowly in informing the public of the dangers.
"It is clear that, during the initial response phase, government communication and information sharing with the general public was not adequate enough to ensure appropriate responses of the affected population," it said in its report on the incident.
Though water supplies have resumed, the melting ice and snow could cause pollution problems to recur in the spring when chemicals thaw, the report said.
"Great care must be taken in the spring when the ice thaws out," the UN body said in a report released on Thursday.
"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.
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.
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 UN 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.
A request to include a public health expert in their investigation team was not accepted, the UN said, asking that the WHO be allowed to help monitor drinking water quality in the region.
UNEP suggested that China work with it to implement an emergency response plan for local authorities who did not disclose that the river had been contaminated until 10 days after the spill was caused by a massive November 13 explosion at a benzene factory.
"Further investigations are needed to clarify whether existing early warning systems and contingency plans were sufficient," it said.
The 80km-long slick of highly toxic and carcinogenic benzene flowed through Harbin along the icy, 1,900km Songhua, which provides the city and surrounding population of nine million with much of its water as well as more than a million across the border in Russia's far eastern Khabarovsk region.