Flood waters are testing the safety of China's massive Three Gorges Dam and raising water levels on its longest river, the Yangtze, after weeks of flooding that have killed about 700 people.

A man stands near stairs on a flooded street in southwest China's Chongqing municipality July 29, 2007. [Reuters]

Water is being released from the reservoir behind the world's largest hydroelectric project through giant sluice gates, raising water levels downstream.

The central province of Hubei has gone on alert as the flood crest is expected to reach or exceed levels that would trigger flood warnings, Xinhua news agency said on Tuesday.

"The Three Gorges Dam has opened 18 sluices and the water level in the reservoir will continue to rise," Xinhua quoted a worker at the dam's operation department as saying.

"The safety of the dam will be tested."

Water is pouring into the Three Gorges reservoir, which stretches for hundreds of kilometers through narrow gorges, at 51,000 cubic meters per second, and could rise to 56,700 cubic meters per second on Tuesday. The sluice gates will release water at 48,000 cubic meters per second.

High water levels forced the closure of the ship locks at the dam on Monday.

Hubei was preparing for flood prevention work along the 1,390-km (860-mile) section of the river that runs from the dam to the industrial city of Wuhan, Xinhua said.

Heavy flooding along the Yangtze in 1998 left the streets of Wuhan waist-deep in water while about 3,000 died and 14 million were made homeless along the river.

About 700 Chinese have died this summer from heavy rains that triggered landslides, floods and house collapses.

Millions fled the swollen Huaihe River in the central province of Henan and the eastern provinces of Anhui and Jiangsu. River levels there have begun to retreat after a month of high water.

In parts of the flood-battered southwestern province of Guizhou, another four people were killed, five seriously injured and three went missing in landslides and mud and rock flows triggered by rainstorms since Sunday, Xinhua said.

The casualties were reported in Renhuai city, renowned for its fiery Moutai drink, China's national liquor, but serious river and street flooding also hit two dozen counties where many thousands were stranded, Xinhua said.

Tens of thousands of passengers were stranded at Beijing airport on Monday when almost all evening flights were cancelled because of hours of thunderstorms and heavy rain, the Beijing News said on Tuesday.

Sixty-nine Chinese miners spent a second day trapped in a flooded coal pit in the central province of Henan, but they were safe and in contact with rescuers late on Monday, state media said.