BEIJING -- Seeking to dismiss lingering doubts, a top Chinese sports official says the country has committed to a zero-tolerance stance on doping ahead of the Olympics in Rio de Janeiro.

Vice Director of the General Administration of Sports Gao Zhidian told the official Xinhua News Agency on Thursday that Olympics-bound athletes and coaches are required to sign pledges not to cheat, saying violators would be severely punished.

Gao said athletes, coaches and team doctors must also pass a written exam on banned substances with only those scoring 80 per cent or more permitted to go to Rio.

"China has been firmly against doping and has a zero-tolerance approach to this particular problem," said Gao, who is also deputy chef de mission of China's 711- member Olympic delegation. "We have made it very clear that we want to ensure fair play and the well-being of athletes."

China has sought to shake off a reputation for doping, particularly in distance running and swimming that gave rise to multiple scandals in the 1990s.

That resolve was questioned when it emerged that officials waited six months before revealing that champion swimmer Sun Yang served a three-month suspension after testing positive in May 2014.

Sun, the gold medallist in the 400- and 1,500-meter freestyle at the 2012 London Olympics, was named male swimmer of the meet at the world championships in Kazan, Russia, last August.

China is sending 416 athletes to Rio, including 35 former Olympic champions. China topped the table at the 2008 Games and won the second largest number of medals in London, including 38 golds.

Based on recent performances by Chinese athletes, some studies predict the country could return to the top of the table at Rio with as many as 39 golds.