The carcinogenic chemical reached the outskirts of Harbin, capital of the northeastern province of Heilongjiang, on Thursday morning, after the city prepared for the potential disaster by closing down the water supplies.

Schools were also closed, while dozens of wells were dug, millions of bottles of water stockpiled and hospitals put on alert for a potential influx of contaminated patients.

Government officials said there had been no reports yet of human contamination, although they conceded the impact of the slick was still unclear.

"Nitrobenzene is very harmful for the environment," Zhang Lijun, vice head of the state Environmental Protection Administration (EPA), told reporters in Beijing without giving any details.

Zhang said it would take about 40 hours for the slick to pass through Harbin, and city officials said on Thursday evening that water supplies may resume in four days.

But the government waited until Wednesday, 10 days after an explosion at an upriver chemical factory which caused the benzene spill, to confirm the potential disaster and some Harbin residents doubted its assurances.

"We have stored up at least 15 days of (bottled) water," Yang Jun told AFP.

A newspaper seller surnamed Cui said he did not trust the government following its failure quickly to disclose the benzene spill in neighbouring Jilin province.

"We don't know what's happening," Cui said as he sat on the banks of the Songhua in below-freezing temperatures.

"They've known about this pollution for 10 days but didn't say a thing, so no one really knows what else they aren't telling us."

Photos of dead fish along the banks of the Songhua did little to ease environmental fears.

Harbin, which has about 3.8 million urban residents and a total population of about nine million, is highly dependent on the 1,897-kilometre Songhua for its water supplies.

In neighbouring Russia, there were growing concerns over the slick, as the Songhua flows into the Heilong river on the two nations' border before supplying the far east Russian city of Khabarovsk.

Russian officials in Khabarovsk said the benzene may pollute the water supplies for 1.5 million people in the region, including 650,000 in the city itself.

Environmental group WWF said in a statement it was "highly concerned that a toxic spill in China will have an adverse impact on the region's people and environment, as well as spreading through the river ecosystem to Russia".

The EPA's Zhang said the slick would likely reach the Russian border in two weeks but played down the impact it would have. "It is weakening and the level of impact will further decrease (by the time it reaches Russia)," Zhang said.

Nevertheless, Chinese foreign ministry spokesman Liu Jianchao said a hotline was being set up between officials from both countries on the issue, adding China had already twice briefed its neighbour.

The contamination was caused by a massive explosion at a PetroChina benzene factory in Jilin province, about 380 kilometers upriver from Harbin.

PetroChina's parent company, China National Petroleum Corp. (CNPC), apologized to Heilongjiang's people over the disaster, Xinhua news agency said.

The company "deeply regrets" the pollution caused and will be responsible for handling the aftermath, CNPC's deputy general manager Zeng Yukang was quoted as saying.

It was still unclear on Thursday how the environmental disaster had affected Songyuan and Zhaoyuan, two other major Chinese cities between Harbin and Jilin that also depend on the Songhua for water.

The government's refusal to publicly acknowledge the benzene spill until Wednesday had fuelled scenes of panic in Harbin over the weekend and early this week when speculation first surfaced of an impending water stoppage.

Hundreds of thousands of people emptied supermarket shelves of bottled water and food amid rumours of an imminent earthquake, while other residents packed their bags and evacuated the city.

A railway official told the Beijing News more people had left Harbin in recent days than during the National Day Golden Week holiday.

On Thursday relative calm had been restored after authorities brought in millions of bottles of water, reassuring residents that at least there would be reliable drinking supplies.

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