HARBIN, China — Premier Wen Jiabao visited this city in China's northeast Saturday and expressed concern for the 3.8 million residents enduring a fourth day without running water while they wait for a spill of toxic benzene in a nearby river to pass.

The government warned residents that water supplies, suspended to protect the city after a chemical plant explosion, would not resume until 11 p.m. Sunday, a full day later than initially planned.

China has been criticized by environmentalists for its slow response to the disaster and is also under pressure from neighboring Russia and the United Nations to release more information about the spill.

Wen's unannounced visit appeared to be meant both as a morale boost to government workers who have been struggling to supply residents with water by truck in subfreezing weather and a warning to local authorities to do all they can to help the public. Local authorities have been criticized already for reacting too slowly to the chemical plant explosion and delaying disclosure to the public.

The premier visited the Harbin No. 3 Water Filtration Plant, where 300 paramilitary police were delivering tons of carbon to filter water from the Songhua River once it is declared safe to use.

"Your work now is work to protect the safety of the masses' drinking water. Thank you, everyone!" Wen told the troops outside the plant, who cheered. "Make the masses' water completely safe, and we must not allow the masses to be short of water."

The disaster is an embarrassment for the government of President Hu Jintao, which has promised to focus on cleaning up environmental damage from 25 years of breakneck economic growth and to look after the well-being of ordinary Chinese.

China's government has also been harshly criticized for its slowness in releasing information on severe acute respiratory syndrome, or SARS, and on bird flu.

Also Saturday, investigators were looking into the chemical plant explosion that the government says dumped about 100 tons of benzene into the Songhua. The government said Friday that officials found responsible would be punished.

Chinese leaders "are paying close attention to this issue and are very concerned about it," chief investigator Li Yizhong was quoted as saying in the newspaper Guangming Daily.

The government's main Xinhua News Agency announced that water service would not resume until 11 p.m. local time on Sunday to make sure supplies are safe.

Tests on the river found benzene levels at Harbin dropped below the official limit early Saturday, Xinhua said. But it said another toxin, nitrobenzene, was still at 3.7 times the permitted level.

Government newspapers on Friday accused local officials of reacting too slowly to the Nov. 13 chemical plant explosion and criticized them for failing to tell the public the truth until this week.

The comments appeared to reflect a high-level effort to prod authorities in Harbin to do all they could to help the public and to warn officials elsewhere to prevent such disasters.

Environmentalists have accused the government of failing to prepare for such a disaster and of not responding quickly enough. They have questioned the decision to allow construction of a plant handling such dangerous materials near important water supplies.

The plant was run by a subsidiary of China's biggest oil company, state-owned China National Petroleum Corp., which issued an apology this week and sent executives to help dig wells in Harbin.

On Saturday, residents of Harbin stood in line in sunny but subfreezing weather to fill buckets and teakettles with water from trucks sent by the city government and state companies. The local government has been sending out such shipments daily, and companies with their own wells have been giving away water to their neighbors.

The disaster has strained China's relations with Russia, where authorities in a city downstream from the disaster complain that Chinese officials have not told them enough about the poisonous benzene headed their way.

The Songhua flows into the Heilong River, which crosses the border and becomes the Amur in Russia, flowing through the city of Khabarovsk.

Russian environmental officials played down the threat Friday, but regional authorities prepared contingency plans including a shutdown of water systems in cities that drink from the river.

A United Nations environmental agency says it has offered to help China with the spill but has received no reply or information about the materials involved or the scope of damage.