Officials from Beijing have been sent to investigate the chemical explosion that contaminated a river supplying water to Harbin in north-east China.
The water system is not expected to be restored until Sunday
The city's water system was shut down four days ago and supply is not due to resume until Sunday, say officials.
Those responsible for the blast at the plant in Jilin would be punished, the government said.
Local authorities have faced criticism for not intervening early enough following the incident on 13 November.
China's leaders "are paying close attention to this issue and are very concerned about it," Li Yizhong, who is leading the investigating team, told local media.
State news agency Xinhua said: "The presence of disciplinary officials on the team indicates that punishment of irresponsible acts are on the way."
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
On Thursday, CNPC, China's biggest oil company which owns the chemical plant, apologised to Harbin's 3.8m residents for the incident.
New filtering material was being installed at Harbin's water plants on Friday to ensure the safety of drinking water once the supply is restored.
An 80km stretch of contaminated water in the Songhua River is passing Harbin.
Tests on the river found benzene levels at Harbin had dropped below the official limit at 0600 on Saturday (2200 GMT Friday), said Xinhua.
But it said a related toxin, nitrobenzene, was still at 3.7 times the permitted level.
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.
Officials are hoping the poisonous chemicals will dissipate as they flow down the river towards Russia.
There has been panic-buying of water in the border city of Khabarovsk, which could be affected by 1 December, according to the Russian authorities.
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.
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