A major Chinese city shut off water to residents to protect the population from possible contamination by a chemical factory accident.

Officials in Harbin, a provincial capital in northeast China, began the water shutdown yesterday and said the extraordinary measure probably would last at least four days. Word of the shut-off, which leaked out before the announcement, sparked panic buying of bottled water by many of Harbin's about 3 million urban residents.

Harbin's plight highlights the environmental challenges afflicting China during its galloping economic growth. The accident that Harbin officials fear may have contaminated their water supply occurred this month after several explosions at a chemical plant owned by a unit of PetroChina Co., one of China's biggest energy companies. PetroChina , majority owned by the Chinese government and listed on the New York Stock Exchange, had suggested in a statement after the incident that environmental damage had been contained.

The company couldn't be reached for comment yesterday.

"Water pollution is a big problem in China," said Ma Jun, an environmental expert and author of the book "China's Water Crisis." "We need to do more work on the risk management of our heavy industry and heavy chemical industry."

Ma said that though he had not studied the details of the Harbin incident, it appeared to be the first time a city of its size had taken such a drastic measure.

The Harbin episode also illustrates the difficulty that Chinese officials face in trying to manage bad news, as technology facilitates the spread of information. Harbin's government initially issued a notice Monday about the shutdown merely stating that the move was to guarantee the safety of the water supply. But by that time, word of the decision had already spread on the Internet, sparking angry Web site postings calling for greater clarity.

The Harbin government then issued a second notice, which local journalists said was not widely distributed until yesterday, acknowledging that the shutdown was triggered by explosions at the chemical factory Nov. 13. The explosions occurred about 125 miles away in Jilin city, near the Songhua River, which flows downstream to Harbin and supplies its water. The plant, owned by PetroChina unit Jilin Petrochemical Co., produces aniline, a toxic chemical used in products including varnish and explosives.

PetroChina had issued a statement Nov. 15 acknowledging the explosion, which it said killed five people and injured 23. The statement said the fire resulting from the explosion "was extinguished after all the substances (toxic substances) on the scene were burnt out." It added that "following an environment survey, it was noted that the air quality in the area of the accident complies with the relevant standard." The statement made no mention of possible water contamination.

The Harbin government, in its second, more detailed notice, said no pollution has yet been registered in the Songhua near the city but that contamination was possible in the near future.

An official in the city's Communist Party office said the plan was to close off the Harbin water supply from the river until any contaminated waters had flowed farther downstream. The official said supply will resume if the water quality improves after four days. "If not, the order will last one or two more days," the official said.

In a new notice posted yesterday, the government urged people to make preparations such as buying bottled water, and it said the city would try to tap underground wells. It also ordered industries that use large amounts of water to shut down.

It wasn't clear how contamination would affect other people who live along the Songhua, which stretches nearly 1,250 miles across the provinces of Jilin and Heilongjiang, home to Harbin.

Pollution has been a growing problem in China, causing water shutdowns before in smaller cities. The central government has tried to step up environmental protection, but its efforts are often foiled by managers at state-run factories and by local officials, experts said.

Residents try to buy bottled water in Harbin, China, where officials have cut off water supply for four days.

A Harbin school is stocked with buckets of water. The Harbin government said no pollution has yet been registered in the river near the city.