China has apologised to Russia for a toxic leak 80km (50 miles) long, which is expected to enter Russia from the Songhua river within two weeks.
The Amur river is a vital resource for Khabarovsk
Foreign Minister Li Zhaoxing's move came after China began an inquiry into how the spill, which caused a major alert in Harbin, was allowed to happen.
Russia's Amur river, which feeds water to more than 500,000 residents of Khabarovsk, is likely to be affected.
Taps in Harbin are due to go back on shortly after a four-day stoppage.
Visiting Harbin, Chinese Premier Wen Jiabao pledged openness about the incident on behalf of the communist government.
He stopped to encourage soldiers and workers who are installing special charcoal filters for the city's water system, Chinese state media report.
Mr Wen also visited a supermarket to check bottled water prices and a university, where he urged students to stay calm.
Foreign Minister Li briefed Russian Ambassador Sergei Razov of the situation on the Songhua - a tributary of the Amur - and the action taken by the Chinese government.
He offered his "profound apologies" to Moscow, Russia's Itar-Tass news agency reported from Beijing.
The spill was "fraught with enormous damage to the natural environment both of China and Russia", he was quoted as saying.
In Khabarovsk, the public has been urged not to panic while the authorities plan to limit the damage from the approaching spill.
As soon as the presence of benzene is detected at the confluence of the Songhua and the Amur, a state of emergency will be introduced in Khabarovsk, Russian public TV reports.
Cold and hot water supplies will be cut off for at least 40 hours and schools, childcare organisations and restaurants will close.
China has promised to punish those responsible for the blast at the chemical plant in Jilin on 13 November which caused the pollution.
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
23 Nov Authorities say 100 tonnes of benzene emptied into the Songhua river
26 Nov China apologises to Russia where the pollution is expected to arrive within two weeks
On Thursday, CNPC, China's biggest oil company which owns the chemical plant, apologised to Harbin's 3.8m residents for the incident.
Tests on the Songhua at Harbin found benzene levels had dropped below the official limit at 0600 on Saturday (2200 GMT Friday), China's state news agency reported.
But a related toxin, nitrobenzene, was still at 3.7 times the permitted level, it added.
Harbin's residents continued to queue up in freezing cold weather to fill buckets and tea kettles with water from lorries. Villagers on the outskirts of the city have been evacuated.
There is also concern about the impact of another chemical factory explosion in the south-western Dianjiang region near Chongqing.
The blast itself, which happened on Thursday, killed one person.
Local schools have been closed, about 6,000 people have been evacuated, and officials have gone from house to house warning residents not to use water from the nearby river.