China has claimed victory in clearing its Olympic sailing venue of a foul-smelling green carpet of algae, but officials will not let down their guard and are patrolling the waters around the clock.

Two barriers of fishnets and canvas, one measuring 22 kilometres and the second 25km, have been laid out to sea off the picturesque coastal resort of Qingdao, where an invasion of algae last month threatened smooth sailing for the Olympic competitors.

More than 1,400 boats and 10,000 troops and volunteers were sent to help clean-up efforts at one stage, moving a jaw-dropping one million tonnes of the algae that produced a pungent odour of rotten broccoli and became enmeshed in the rudders of international teams practicing for the August 8-24 games.

Soldiers had been racing against time to scoop up the bloom with shovels, rakes and their bare hands, creating piles of woolly green clumps on the shore.

"There's so little algae out there now that I really don't think it'll be a problem," said Morgan Reeser, a coach with the British sailing team. Thirty-seven national teams are now training at the site, he said.

Wang Wei, vice president of the Qingdao Sailing Committee, said: “Yesterday, we sent out 1,700 fishing boats to search for algae over the whole sea area in Qingdao, but they only got 10,000 tonnes. Compared with days before, that's big progress.”

Qingdao authorities warned officials not to be complacent, however, and to be ready to attack further outbreaks. "We cannot slack off for even a second. We must strive for victory and keep clear-headed at this time," the city government said on its website.

There was no sign of complacency. Locals said even the tiniest patch of algae could multiply several times within hours and volunteers are patrolling the coast and sea to report in all outbreaks. As soon as a green clump is seen, navy boats and planes are dispatched to deal with the offending mass.

Officials have been at pains to cast the algae bloom as a harmless natural phenomenon, but local residents and scientists have expressed doubts, blaming industrial pollutants and agricultural run-off for feeding the bloom.