Hours after respected American swimming coach John Leonard raised concerns about Chinese swimmer Ye Shiwen's formidable performance in the pool, China's anti-doping chief hit back, at the "biased" suspicions.

"I think it is not proper to single Chinese swimmers out once they produce good results," Jiang Zhixue, the anti-doping chief at China's General Administration of Sport, told state news agency Xinhua. "Some people are just biased.

"We never questioned Michael Phelps when he bagged eight gold medals in Beijing."

Jiang claimed the Chinese team had improved due hard work and a scientific approach to training. "The Chinese athletes, including the swimmers, have undergone nearly 100 drug tests since they arrived here.

"Many were also tested by the international federations and the British anti-doping agency. I can tell you that so far there was not a single positive case."

Deputy anti-doping chief Zhao Jian also came out in support, saying throwing suspicion on an athlete without any evidence was unfair. "It seems in the sports world people always suspect good scores. You cannot assume a runner is not a normal person just because he runs faster. Those assumptions are not fair to any athlete."

Several Chinese swimmers have fallen prey to doping in the past, with seven testing positive for steroids at the Asian Games in 1994. Four years later four athletes were sent home after a bag containing banned growth hormones were found on a swimmer, during a routine check up in Australia.

"In 1990, when China started the doping test, we did 65 tests. Right now, we do 15,000 tests each year. As far as I know, American and Australia carry out 7,000 to 8,000 a year," Zhao said.

"All Chinese athletes get anti-doping education and training, take an oath and take an exam. Our system is serious and severe."

The controversy in London erupted when the executive director of world swimming, John Leonard, called Ye's performance "unbelievable, disturbing and suspicious", while saying it brought back memories of Irish swimmer Michelle Smith's race in Atlanta 1996 and old East German swimmers. Smith, now Michelle de Bruin, was handed a four-year ban after testing positive for an anabolic steroid in 1998.

Ye put in a stunning performance in the 400 meter individual medley on Saturday, setting a world record time. Her final 100 meters in freestyle, in 58.6 seconds, particularly caught the eye; in the final 50 meters, she was even faster than the split time of American Ryan Lochte, the winner of the corresponding men's event.