In June, large swathes of offshore waters became clogged by the unsightly green algae, disrupting training for Olympic sailors and prompting an intensive clean-up operation involving 1,400 boats and 10,000 troops.

"Outside the area of Qingdao, there are still some areas of algae but it will not disturb the competition," added Wang.

Organisers are using containment booms, vertical nets and regular reconnaissance missions to make sure any algae does not drift back into the competition areas.

"We have the eyes of an eagle and the speed of a puma and have worked really hard to fight this," said Wang.

American Star sailor John Dane said conditions had greatly improved.

"Yesterday we didn't see any algae. It's the cleanest day we have had," he told Reuters.

"We were here for two days in June and it was really bad. We came back in mid-July and it was improved. Yesterday, only one time did we see a string of algae on our keel and that is such a change."

Qingdao, in northern China's Shandong province, is approximately 450 miles (720 kms) southeast of Beijing. The competition begins on Aug. 9 and runs through to Aug. 21, with two reserve days set aside.

(Reporting by Justin Palmer; Editing by Ed Osmond)