A 50-mile-long slick of toxic river water moved slowly through this industrial city in China's frigid northeast on Friday, as government officials fended off questions about their slow and secretive response to a chemical spill and millions struggled with the third day of an emergency shutoff of the municipal water supply.

Harbin's mayor, Shi Zhongxin, vowed on the front page of the local newspapers that the city would resume pumping water by Monday at the latest, though he warned that the supply could be intermittent and unsafe for drinking for several more days.

He said the government was strengthening its filtering system and drilling 100 deep wells to keep minimal amounts of safe water flowing to schools, hospitals and the water-based heating systems that keep most people warm here. Officials trucked in more drinking water in plastic bottles and froze prices at 12 cents a liter to avoid gouging.

The crisis dramatized the threat to China's environment created by booming economic growth and, officials acknowledge, frequently inadequate enforcement of anti-pollution and industrial safety rules. A recent government report said that up to 70 percent of China's rivers and lakes have become unacceptably polluted over two decades of swift expansion.

The pollution, which officials said included benzene and nitrobenzene, was in an estimated 100 tons of chemicals that emptied into the Songhua River after an explosion Nov. 13 at the Jilin Petrochemical Co. that killed five workers and injured 70 more. Since then, it has flowed about 165 miles northward, and at about 5 a.m. Thursday reached the point where Harbin's water pumps suck in supplies.

From above Harbin, the river looked like a brown ribbon, with multiple streams flowing around dry farm fields. There was no apparent sign of the colorless pollutants. Large sections of the river were frozen, at least on the surface, which experts said would slow the dispersal and evaporation of the slick.

Zhou Liqun, 35, a cab driver in the city, was among residents who expressed anger. "The chemical factory should be punished for all the trouble it has brought us," he said. "We can't take showers. Because we can't wash dishes, we're eating out of plastic bags."

He said his family had stored enough water to last five or six days in basins, bowls and bottles. "I don't believe the water quality will be normal after four days," he said. "I'll keep buying bottled water."

Other residents said they believed the government's promise but were being careful anyway. "It's not that serious. We've been drinking water from the Songhua River for decades," said Zhu Hong, 40, a physician who was at the airport with her 12-year-old daughter preparing to leave for Beijing.

The State Environmental Protection Administration estimated that it would take about 40 hours for the polluted section of the river to pass through Harbin, a city of 3.8 million people in Heilongjiang province more than 600 miles northeast of Beijing. But other communities downstream also seemed likely to suffer contaminated water supplies in the days and weeks ahead.

Zhang Lanying, director of the Environment and Resource Institute at Jilin University, told the official New China News Agency that nitrobenzene is a particularly toxic pollutant that, if ingested in large amounts, can foster development of leukemia. No immediate cases of illness have been reported.

Harbin officials announced they had dumped a carbon-based powder into the river to counteract the pollution, but it was unclear how effective that could be.

The Russian city of Khabarovsk, which sits on the border with China, said China's pollution would soon reach its drinking water supply as well. But Chinese officials said it would take at least two weeks for the polluted waters to reach that area.

The Foreign Ministry said it had informed the Russian government of the threat. A spokesman, Liu Jianchao, said the ministry had briefed the Russian Embassy in Beijing twice so far this week.

The Chinese National Petroleum Corp., which owns and operates the Jilin chemical plant, issued an apology to the people of Harbin. The deputy general manager of the giant state-owned enterprise, Zeng Yukang, said his workers would help Harbin officials with emergency drilling for the wells to take the place of normal water supplies.

But several Harbin residents complained to reporters about the way the crisis was handled. The State Environmental Protection Administration acknowledged only Wednesday that the city faced a major pollution threat and identified the chemicals in the water, they noted. In addition, they said, the city cut off water supplies Wednesday morning, then reopened the pipes briefly in the afternoon to allow people to stock up in their bathtubs and plastic containers, only to shut off the flow again during the night before the polluted part of the river streamed by the municipal intake pumps.

But Zhang Lijun, deputy director of the environmental agency, said at a news conference in Beijing that the crisis was handled properly. He said authorities in Jilin immediately notified officials in charge of downstream areas, including Harbin and Heilongjiang province, that the pollution was on the way and that those officials swiftly began planning.

But Harbin residents learned of the danger only as it approached their city. Thousands of them went to the train station and airport to leave Tuesday and Wednesday. Thousands more lined up at stores selling bottled water or drove to relatives' homes in outlying areas where water does not come from the river.

Xue Ye, general secretary of Friends of Nature, a Chinese environmental group, said the government should have protected the river from pollution in the first place by requiring adequate safeguards at the plant in Jilin. The cause of the blast there has not been determined.

Cody reported from Beijing.

A Chinese firefighter gives water to a local resident in Harbin, the capital of Heilongjiang province, where officials have cut off the water supply.