Harbin, China ? Experts warned Tuesday that dangers from a huge chemical spill in this northeastern Chinese city could last for years because of toxins – including cancer-causing benzene – embedded in ice and mud at the bottom of the Songhua River.

Their concern came as city officials in Harbin and down river in Russia’s Far East, where the 50-mile-long chemical slick was headed, sought to reassure residents their water was clean.

“Harbin’s water is now safe to use and drink,” Xiu Tinggong, vice director of the city’s health inspection bureau, said on local state television. “Everybody can rest assured.”

In Khabarovsk, Russia, a top environmental official drank a glass of tap water on television to show his confidence in its purity. Officials estimate the benzene spill flowing from the Songhua into the larger Heilong River, called the Amur in Russia, should reach the border city around Dec. 10-12 – or sooner.

Chinese Health Minister Gao Qiang said the incident highlighted a “major problem.”

Water was shut off for five days in Harbin, the capital of the northeastern province of Heilongjiang after the Nov. 13 explosion at a nearby chemical plant. The blast, which authorities said killed five people, spewed 100 tons of benzene and related toxins into the Songhua, which passes through Harbin and provides most of the city’s drinking water.

People get drinking water from a street water pump Tuesday in Khabarovsk, Russia. Toxins from an explosion at a Chinese chemical plant could continue to pollute rivers for years to come, experts warned as the chemical slick continued to move toward Russia.

In Russia, the Emergency Situations Ministry said the pollutants could affect 70 Russian cities and villages with a total of over 1 million residents along the Amur River. A spokesman for the World Wide Fund for Nature said the river faced “ecological catastrophe” from the chemical slick.

“There will be an effect on nature – plants and fish will die – and economic damage,” said Ilya Mitasov, a Moscow-based spokesman for the environmental group.

The only way to get rid of the toxins is evaporation, but water temperatures would have to be 68 to start that process, Mitasov said. Currently it’s about 50 degrees, and there is ice on some stretches of the Amur River, which ultimately feeds into the Sea of Okhotsk.

Zhang Qingxiang, professor of environmental studies at Shanghai’s East China University of Science and Technology, also warned that the Songhua’s spring thaw could bring another wave of benzene contamination.

Authorities “should pay much attention next spring when the ice is going to melt,” Zhang said.