BEIJING - Chinese sports authorities warned their coaches and athletes to stay away from any attempts of doping in their preparation for the London Olympic Games.

China's General Administration of Sport on Wednesday summoned heads of  associations of Olympic sports, training bases and provincial sport bureaus to sign a "responsibility agreement" and to commit to the anti-doping drive, sportsmanship and good discipline at the London Games.

The administration's head Liu Peng said China has been a "vigorous fighter" against doping but the challenge should not be underestimated.

"China has made increasing efforts in cracking down on doping in the past years, including an improved legal system, effective doping tests and enhanced education," said Liu. "Therefore China has effectively taken the number of positive cases under control."

"But we are aware that the fight against doping will be a long and painstaking one. We are in a tough situation in terms of anti-doping, which remains a threat to the Chinese delegation," he added.

China Anti-Doping Agency (CHINADA) last Saturday announced four positive cases, including one involving swimming world champion Li Zhesi.

Li, turning 17 in August, was found positive for performance-enhancing drug erythropoietin (EPO) in an out-of-competition test in March.

The winner of 4x100m medley relay in 2009 worlds has asked for a hearing after the surprise announcement by CHINADA. Positive cases were usually unveiled by relative sport associations after hearings were held and punishments decided.

The new measure is expected to give the relative association a nudge to speed up their investigation.

The positive case may deal a blow to the Chinese swimming, which strived to rebuild its fame in the past few years with Zhang Lin winning Chinese men's first Olympic medal in Beijing in 2008 and the first men's world title in Rome in 2009. Sun Yang succeeded Zhang to retain the 800m freestyle title in the 2011 world championships in which Ye Shiwen, Zhao Jing and Jiao Liuyang also triumphed in women's events.

The other three positive cases were from athletes who are not London-bound. They are from events including rowing, athletics and tug of war.