In a sign of how the spill has jarred national nerves about widespread pollution, Prime Minister Wen Jiabao issued instructions demanding safe drinking water be ensured.

In Heilongjiang, Governor Zhang Zuoji ordered hospitals to brace for possible cases of poisoning and promised to drink the first glass of water from city taps once the pollution passes.

According to initial estimates, the explosion resulted in 100 tonnes of benzene and related products being released into the water, deputy head of the State Environmental Protection Administration, Zhang Lijun, told a news conference.

A provincial government spokesman said the 80-km (50-mile) stretch of pollution passed Harbin's water supply inlet early on Thursday and would flow beyond the city on Saturday.

Water supplies could resume partially as early as Sunday, Xinhua said. Harbin's mayor, Shi Zhongxin, said the water would at first be unsuitable for drinking.

"WE CAN'T TELL OURSELVES"

Residents' reactions ranged from stoic acceptance to anxiety, but there were few signs of panic in Harbin, where most residents continued to work and shops and restaurants remained open.

"It's worrying, because it may not have a strong smell or colour, so you can't tell when it's gone," said Hong Shan, a retired official exercising beside the river. "It's up to the government to keep us informed. We can't tell ourselves."

Commentators in Beijing and further afield condemned the "lies" told before the authorities revealed what had really happened. A paper in Harbin itself tried to play down the crisis.

Farmers in surrounding areas mostly said they draw water from wells, and so were not panicked by the spill.

"We've stored up enough water to get by, but I don't know if this pollution can seep into the underground water," said Gao Erling, from Sifangtai Village near Harbin.

The explosion happened at a chemicals plant in neighbouring Jilin province about 370 km (220 miles) from Harbin on Nov. 13.

The plant was only a few hundred metres (yards) from the Songhua, but at the time officials there warned only of air and ground contamination, not water pollution. Five people were killed in the blast.

Across China on Thursday, a blast shook a chemical factory in the southwestern province of Sichuan, killing one person, injuring three and raising fears of benzene contamination in the nearby town of Danjiang, Xinhua news agency reported.

The Jilin plant, Jilin PetroChemical Co., had insisted it was not responsible for the pollution, state media said. But the deputy general manager of China National Petroleum Corp., Jilin PetroChemical's parent company, apologised to Harbin residents.

China's environment administration said on Thursday the plant should be held responsible for the toxic spill, Xinhua said.

The pollutants had already passed through the smaller city of Songyuan, between Jilin and Harbin, where water supplies had been partially cut for seven days.

A Harbin environmental protection group recently issued a report documenting widespread chemical pollution along the Songhua River.

The report said that many factories were secretly dumping waste water and chemicals into the river, and water treatment plants could not do enough.

"Overall, the water quality along the Songhua has improved in recent years, but there are problems with factories and pulp mills releasing untreated waste," said Zhang Yadong, one of the authors. "People can suffer, but it's fish that are worst hit."

Russia's environmental protection agency said it was worried the pollution might affect drinking water in its Khabarovsk region, which the Songhua enters several hundred km downstream from Harbin.

The State Environmental Protection Administration's Zhang rejected accusations that the local authorities had waited too long before telling residents or Russia about the pollution.