The Chinese central government sent a team of investigators and "disciplinary officials" to Harbin today to study the handling of a toxic spill that has led to the shutting down of the city's water supply.

As the 80km-long (50-mile) stretch of benzene-polluted water continues to slide down the Songhua River and through the city, the decision to send the officials from Beijing "indicates punishments of irresponsible acts are on the way", according to the official Xinhua news agency.

Today was the fourth day that Harbin, the capital of the northeastern province of Heilongjiang and home to four million people, has been without water since the city's taps were turned off without explanation earlier in the week.

Two days ago, provincial officials admitted that an explosion at a chemical plant on November 13 had leaked quantities of benzene, a carcinogen, into the Songua River. Yesterday, officials conceded that around 100 tonnes of the toxic solvent, which causes leukaemia in high doses, had been released.

The slow trickle of news, which led to the panic-buying of water in Harbin before the government sent water trucks and fixed the price of bottled water in the city, has led to an unprecedented criticism from the Chinese media.

"The incident... has stunned the whole nation. It has been horrible for more than 3 million people of Harbin to be bereft of water for four days," the state-run Xinhua news agency wrote today, adding that the spill should prompt environmental reform in China, where industrial pollution has ruined waterways across the country.

"Conditions for the change are all there. It needs only a little more push, followed by focused and well-organized efforts," said Xinhua. "World environmental protection equipment manufacturers should keep an eye out for that."

A column in the China Daily newspaper was even more direct, admonishing local government officials and managers of the China National Petroleum Corporation, which runs the chemical plant where the accident took place, for "the hiding of truth".

"We do not know what is behind the cover-up. It might be because they were afraid that they would have to pay money for the losses the pollution has incurred in Harbin, and it might be because they were afraid of losing face. But the fact is they have brought shame on themselves by covering up the truth," the newspaper said.

Teams of soldiers and emergency workers today started to release 1,200 tonnes of activated carbon into Harbin's water supply, to help filter and absorb the benzene. Contamination in the Songhua River has fallen during the week but remain 17 times above acceptable levels.

"People I've spoken to say that they trust the government's efforts to clean the water," said Jane Macartney, The Times China Bureau chief, who is in Harbin. "They have been told to run the water for a few hours when the taps get turned back on before drinking it."

The UN repeated its offer to help China clear up the spill today but said it was yet to receive sufficient information to launch a response. "We need basic, official information from the Chinese side, which we do not have," said Vladimir Sakharov, who heads the Environmental Emergencies Section of the UN's Environment Program. "We have asked, we are still waiting," he said.

Concern is also increasing downstream of Harbin, where the Songhua River becomes the Amur River and crosses the border into Russia. Although scientists expect the contamination to dilute as it spreads through the river system, it will also become more difficult to control.

The slick of chemicals is expected to arrive in the Russian city of Khabarovsk on December 4 or December 5. Today, the Russian branch of the World Wildlife Fund complained about the lack of clear information and said that it believed benzol, another carcinogen, was among the pollutants in the river.

"There is a severe shortage of information from the Chinese officials about the type of chemicals contaminating the rivers. The coordination of the Russian and Chinese officials is poor", said Alexey Kokorin, the charity's toxic programmes coordinator.

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