China's Three Gorges Dam could trigger an environmental catastrophe unless emerging problems are treated urgently, senior officials have warned.
China says the dam will help power its economic growth
The dam's head of construction, Wang Xiaofeng, said ecological problems like soil erosion, landslides and water pollution could not be ignored.
In some areas ill-judged development was making things worse, he said.
Critics have long warned the dam, the world's largest hydro-electric project, could cause huge environmental damage.
The $25bn (£12.5bn) project, across the country's biggest river, the Yangtze, is due to be completed by the end of 2008.
More than one million people were relocated to make way for the dam, which China says is needed to control flooding and provide much-needed electricity.
Mr Wang told a conference that China had to address the environmental issues.
More than one million people were relocated because of the dam
"We absolutely cannot relax our guard against ecological and environmental security problems sparked by the Three Gorges Project," he said.
"We cannot win passing economic prosperity at the cost of the environment," Xinhua news agency quoted him as saying.
The problems included landslides caused by erosion on the steep hills around the dam, conflicts over land shortages and "ecological deterioration caused by irrational development", he said.
Senior engineer Huang Xuebin told the forum that landslides were a "severe threat to the lives of residents around the dam".
Some landslides had caused waves several metres high that further damaged surrounding shores, he said.
Other officials warned that the quality of drinking water for residents was being affected.
The BBC's Quentin Somerville, in Shanghai, says that the admission comes with China's government increasingly worried that environmental damage is leading to growing political unrest.
Earlier this summer, the head of the State Environmental Protection Agency warned that pollution worries had led to an increase in protests and riots across China.
However, there is hardly a river in the country that has not been dammed and many more projects are still progressing, our correspondent says.
Beijing recently increased its targets for renewable energy production, most of which will still come from hydro-electric projects.
THE THREE GORGES DAM
Type: Concrete Gravity Dam
Cost: Official cost $25bn - actual cost believed to be much higher
Work began: 1993
Due for completion: 2009
Power generation: 26 turbines on left and right sides of dam. Six underground turbines planned for 2010
Power capacity: 18,000 megawatts
Reservoir: 660km long, submerging 632 sq km of land. When fully flooded, water will be 175m above sea level
Navigation: Two-way lock system became operational in 2004. One-step ship elevator due to open in 2009.