A state of emergency is being imposed in the city of Khabarovsk in Russia's far east, as a toxic spill heads towards the Amur River from China.
Khabarovsk relies on the Amur river for water
A senior Russian official told the BBC that the mains supply of drinking water - taken from the Amur - would be cut off for several days.
A communications hotline has been set up between Chinese and Russian experts to monitor the benzene spill.
The city of Harbin in north-eastern China is relying on bottled water.
Khabarovsk is home to about 650,000 people and the authorities fear 1.5 million people in and around the city will be affected by the pollution.
Oleg Mitvol, deputy head of the Russian environmental monitoring agency Rosprirodnadzor, told the BBC that in Khabarovsk "the water supply will be shut off, because the purifying equipment cannot deal with benzene".
He said heating would not be affected but tap water would be cut for a few days and fish from the Amur would also be contaminated.
13 November Explosion at petrochemical plant, Jilin city
21 Nov Water to Harbin city cut off; local government cites mains maintenance
22 Nov State media say water could have been contaminated after the blast
23 Nov Authorities admit very high levels of benzene have been found in the water
"We expect the spill to arrive in Russia on 26 November and in Khabarovsk on 30 November-1 December," he said.
However, Chinese officials said they expected it to take two weeks to reach the Amur river.
The toxic swathe of water has drifted down the Songhua River, which flows into the Amur.
Massive amounts of the chemical benzene were released by an explosion 11 days ago at a chemical plant in Jilin.
A BBC correspondent says Harbin seems to be well stocked with bottled water.
Reports on Thursday said the spill had levels of benzene more than 30 times higher than is considered safe.
A US citizen living in Khabarovsk, Patrick Perner, told the BBC News website that residents had been told to stock up with two weeks' supply of water.
"I am told that the entire city has sold out of water and plastic containers. Everybody is filling their bathtubs with water. It's typical hoarding behaviour.
"People have been orderly and calm, it's only that the shelves were empty of water," he said.