The United States said Tuesday it was sending experts to help China limit damage from a toxic spill in a river, while Beijing promised to work closely with Moscow to cope with the chemicals flowing toward Russia's Far East.

Meanwhile, the American ambassador to China expressed concern that Beijing failed to react quickly enough to the spill, which has disrupted water supplies to millions of people.

The expanded cross-border efforts to limit damage came as this city of 480,000 people in China's northeast rushed a new water plant into operation following the shutdown of another facility near the river for fear of contamination.

U.S. Ambassador Clark T. Randt said Beijing accepted Washington's offer to send a team from its Environmental Protection Agency.

"We are sending to China an environmental impact assessment team," Randt told a group of business leaders in Hong Kong. He said they would "help make suggestions on remediation efforts."

Beijing has apologized to Moscow for the spill caused by a Nov. 13 chemical plant explosion. It is expected to reach Khabarovsk, a border city with 580,000 people, early next week.

In a letter this week to his Russian counterpart, Chinese Premier Wen Jiabao "reaffirmed China's readiness to further cooperate with Russia to deal with the aftermath of the pollution," said Qin Gang, a Chinese Foreign Ministry spokesman.

Randt also expressed concern at China's slow response.

"It seems it took a while to come to the attention of the central government and of course, that's a bit worrisome to us because we remember SARS and how that took a little while to come to people's attention," he said, referring to China's outbreak of severe acute respiratory syndrome in 2002-03.

Chinese media have accused local Communist Party leaders of trying to conceal the spill.

The government didn't confirm that the Songhua was tainted with benzene until Nov. 23, 10 days after the accident and just before the chemical reached Harbin, a city of 3.8 million people.

Harbin shut down running water for five days, forcing residents to stand in sub-freezing weather to get water from tanker trucks and hurting hotels, restaurants, factories and other businesses.

The director of China's environmental protection agency has resigned and the general manager of the chemical company blamed for the spill was removed from his post. But there has been no sign that Communist Party officials might be punished.

The spill reached Jiamusi early Tuesday, said a spokesman for the Environmental Protection Office of Heilongjiang province, where the city is located. He would give only his surname, Wu.

The concentration of nitrobenzene, one of the toxins in the slick, were more than eight times the safe level, the official Xinhua News Agency said.

The chemical slick, now some 90 miles long, has been lengthening and slowing as the Songhua freezes over.

In the first details about what is expected to be the disaster's huge economic impact, a news report said the government of Harbin is borrowing 640 million yuan (US$79 million; euro68 million) to pay for recovery efforts.

The report by the state-run Web site Northeast Net didn't say what the money would be used for.

Harbin and other communities on the river brought in tanker trucks to supply residents with drinking water. The government hasn't said whether businesses, farmers and fishermen might receive compensation.

On Friday, Jiamusi shut down three of the seven wells that feed its main water plant, saying they were close to the river and that it wanted to avoid contamination.

The city's new Jiangbei Water Source, built at a cost of 230 million yuan (US$28 million; euro24 million), is capable of producing 100 million liters (26 million gallons) of water per day, the local party newspaper Jiamusi Daily said.

It was rushed into operation because of the chemical spill, and the government at one point had 1,500 people working around the clock to complete construction, the newspaper said.

"We're ready. Control and prevention work has entered a critical stage," said Mayor Li Haitao, quoted by Xinhua. "We'll do whatever we can to ensure drinking water safety and maintain normal, stable social order in urban as well as rural areas," he said.

Daily output at Jiamusi's main water plant has fallen from 130 million liters (34 million gallons) to 20-30 million liters (5-8 million gallons), said Wang Li, its deputy general manager.

Wang said the plant supplied up to 80 percent of the city's running water, serving some 600,000 people in Jiamusi and surrounding areas.

"It will be hard for the contamination to get into the ground water, but we will keep testing," Wang said.