Beijing - The chairman of the World Anti-Doping Agency (WADA) admitted Friday that athletes who lie about their age remain almost impossible to catch but said the low number of doping cases at the Olympics so far was a sign of a change of attitude to doping.

On the day that North Korean marksman Kim Jong Su became the first athlete to lose an Olympic medal for doping, Professor Arne Ljungqvist said it was a positive sign that this had happened seven days into competition.

"I've not expected that many cases to occur during the Olympic Games, we usually have 12 cases or so. But this time many federations have conducted intensive out of competition testing before the Games and we have seen the consequences of that," the Swede told a news conference.

"It's encouraging and shows that hopefully the athletes here competing are clean."

In the run-up to the Olympics, athletics' ruling body, the IAAF, provisionally suspended seven female Russian athletes, including middle-distance runner Yelena Soboleva, for allegedly tampering with their urine samples.

However, Ljungvist was not so optimistic about catching athletes who lie about their age, a problem most contentious in women's gymnastics where controversy has raged over the true age of some of the Chinese competitors who secured team gold for the host nation on Wednesday.

He Kexin is 16, the minimum age for Olympic eligibility, according to her passport but Chinese media has reported her as being as young as 14, while Jiang Yuyuan was also reported as being under 16 in a list of junior competitors from the Zhejiang Province sports administration.

"This is a problem when you have an age limit that there is a temptation for manipulation. How to prove this is scientifically very difficult," said Ljungvist.

"There are ways but it is not a scientifically or legally accurate way of doing it. It can be plus or minus two years and can be manipulated using certain substances."

The age issue apart, the International Olympic Committee (IOC) revealed there have been 2,203 doping tests to date, with 1,250 of those coming before the Olympic Games started, with Kim the first to lose a medal as a result of a failed test.

Kim won bronze in the men's 10-metre air pistol last Saturday and followed that up with a silver in the 50m pistol on Tuesday but loses both after testing positive for the beta-blocker propranolol.

"He has been excluded from the Games and disqualified," IOC communications director Giselle Davies said, following Friday's IOC Executive Board meeting on the issue.

Jong's disqualification means Tan Zongliang of China takes silver in the 50m and Russia's Vladimir Isakov is promoted to bronze.

In the 10m air pistol, American Jason Turner now takes bronze.

Vietnamese gymnast Do Thi Ngan Thuong, who finished 59th in qualifying for the women's all-around gymnastics, also failed a dope test and was disqualified but Ljungqvist, said this was more likely due to error.

"This is probably a result of poor information given to the athlete involved," he said.