Bi Xiaogang, deputy director of the Beijing Water Authority, said officials had studied and adopted measures to prevent algae outbreaks in preparation for the Games for a number of years.

"I can responsibly say that all of the waters at Olympic venues will not develop algae outbreaks, during and after the Games," Bi said.

Algae blooms develop in water rich in nutrients, often because of run-off from heavy fertiliser use, chemical pollution, or untreated sewerage, all pollutants in ready supply in many parts of China.

Beijing officials last year were on high alert after summer heat and low rainfall threatened to cause blooms, similar to ones in southern China that cut off drinking water to millions of people.

In Qingdao, officials have set a deadline of July 15 to banish the algae from coastal waters, and have ordered nine provinces to build a 32-kilometre (20 miles) marine fence around the sailing venue, Xinhua news agency said on Wednesday.

About 170,000 tonnes of algae have already been scooped from waters and beaches in Qingdao, where about 30 countries are currently training for sailing events, Xinhua said.

The former German concession port and popular summer resort for millions of Chinese is regularly blighted by algae outbreaks.

Bi also promised Beijing's water supplies would be enough to meet the needs of an extra 2.5 million Games visitors, after Probe International, a Canadian-based conversation group, condemned the city's tapping of strained underground supplies to provide water for Olympic beautification projects.

Beijing's thirst for Olympic water has also seen the construction of a mammoth 309-km (192 mile) canal to pump emergency reserves from neighbouring Hebei, already one of the country's most water-short provinces.

"I can tell you that water reserves in Beijing's two (main) reservoirs are enough to ensure the supplies during the Olympic period", Bi said, but would not rule out the city tapping water from its arid neighbour.

"We will study the water consumption situation in Hebei and Beijing ... and if there is a major impact on agriculture in Hebei, we will provide compensation to local farmers," Bi said. (Reporting by Ian Ransom; Editing by Alex Richardson) (For more stories visit our multimedia website "Road to Beijing" at http://www.reuters.com/news/sports/2008olympics; and see our blog at http://blogs.reuters.com/china)