The International Olympic Committee has conducted 650 drugs tests since the start of the Olympic period and not yet recorded a positive test, IOC Medical Director Patrick Schamasch said on Thursday.

The IOC took over anti-doping controls from the World Anti-Doping Agency on July 27, the official opening of the athletes' village, and will run tests until the end of the Games.

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"We have conducted about 650 tests so far and there has been no adverse analytical finding yet," Schamasch told a small group of reporters.

He said the tests, both on blood and urine samples, were conducted in all Olympic venues as well as training camps across the globe.

"They were conducted all over the world," Schamasch said. "Mainly within the Olympic venues and training centers as well as Singapore, Hong Kong, United Kingdom, everywhere."

The IOC will carry out about 4,500 doping tests in Beijing [Images], the largest number at an Olympics [Images], in an effort to root out drugs cheats who have in the past editions of the Games tarnished the image of the Olympics.

Some 10,500 athletes are taking part in the Games.

Tests include a new and improved test for the once undetectable human growth hormone (HGH).

IOC President Jacques Rogge said based on the number of tests and positive cases at the Athens 2004 Games, he expected some 30 to 40 positive cases in Beijing.

"If we have less I would be extremely glad because that would mean that with the same number of tests we would have (comparatively) less cases than in Athens," Rogge told reporters.

"I count on the deterrent effect," he said.

"I hate doping. But we have to be realistic and not utopian. Doping is to sport what criminality is to society."

He said it was the IOC's obligation to take all necessary measure to ensure a level and clean playing field for all athletes.

Rogge said all samples taken in Beijing would be frozen for eight years to allow for retroactive testing for substances that now potentially could go undetected.

The IOC will test the top-five finishers at every event as well as several other athletes at random. It will also conduct targeted tests if and when it receives information of suspicious athletes or team officials, it has said.

The IOC also urged international federations and national Olympic committees to increase their own tests before the Games to help enforce the IOC's "zero tolerance" on cheats and have only clean athletes at the Games.

Dozens of athletes have failed tests in the weeks and months leading up to the Aug 8-24 Olympics.