BEIJING -- China's government rushed Thursday to shield the country's southern business center, Guangzhou, from a toxic spill of cadmium flowing toward the city of 7 million -- the second manmade disaster to hit a Chinese river in six weeks.

Meanwhile, a slick of toxic benzene from the first accident in the north arrived in the Russian city of Khabarovsk, where worried residents flooded a telephone hot line

The twin disasters highlight the precarious state of China's water supplies for industry and homes. Regulators say its major rivers are badly polluted and millions of people lack access to clean water.

Authorities in southern China were dumping water from reservoirs into the Bei River to dilute the cadmium spill from a smelter.

Cadmium is a soft, bluish-white metal found in lead and zinc ores. Exposure to it can cause lung and prostate cancer, kidney damage and bone disease, according to the U.S. Occupational Safety and Health Administration.

The government did not say when the smelter spill would reach Guangzhou, the heart of the region near Hong Kong with factories supplying China's booming export industries. But the official Xinhua News Agency said city leaders were ordered to "start emergency plans to ensure safe drinking water supplies."

Six weeks ago, a chemical plant explosion spewed benzene and other toxins into a northeastern river, disrupting water supplies for millions of people and straining relations with neighboring Russia.

On Thursday, the benzene spill flowed into the city of Khabarovsk in Russia's Far East. Authorities there said the city of 580,000 people would keep supplying running water from the river because chemical levels were still within a safe range.

The smelter spill in the south already forced two cities upstream from Guangzhou to stop using river water, according to state media.

Yingde, a city of 210,000 people, stopped using Bei River water late Wednesday, switching to water drawn from a nearby reservoir through a hastily built pipe, Xinhua said.

Farther upstream, the city of Shaoguan -- where the spill occurred -- suspended running water for eight hours Tuesday.

"The density of cadmium kept dropping after the local governments began diluting the polluted water by increasing the discharge of the water reservoirs at the Bei River's upper reaches," Xinhua said.

"Experts forecast that the diluted water will likely not threaten the drinking water source for the downstream cities of Foshan and Guangzhou."

Authorities were preparing to release 200 billion gallons of water from a Shaoguan reservoir to dilute the chemicals, the China Daily newspaper said. It did not say when that would happen.

The Bei flows into the Pearl River, which passes through Guangzhou and some of China's most densely populated areas before emptying into the South China Sea just west of Hong Kong.

There was no indication whether industry might be affected.

A woman who answered the phone at the media relations office of Guangzhou's water department refused to say whether it had cut running water or how many people might be affected.

However, she said only one of Guangzhou's seven water plants is close enough to the river to be affected by the pollution. She would give only her surname, Zheng.

Phone calls to the city government and environmental bureau were not answered.

In the northeast, the Songhua River benzene spill forced the mayor of Harbin to shut down running water to 3.8 million people.

A Chinese Foreign Ministry spokesman defended Beijing's handling of the benzene spill, saying Thursday it was trying to minimize the impact on Russia.

Beijing has sent Khabarovsk tons of activated carbon for water filtration and built a dam meant to keep the toxins out of another river used by the city.

"We express our regret for the possible impact and difficulties the Russian side may have in dealing with this issue," said Foreign Ministry spokesman Qin Gang. "But we have been very quick to respond and to take measures to prevent or to minimize the pollution's impact in our cooperation with Russia."