MOSCOW (REUTERS) - A Moscow laboratory used for doping tests has stopped operating after its accreditation was suspended by the World Anti-Doping Agency (WADA), the head of the Russian Anti-Doping Agency, Mr Nikita Kamaev, said on Tuesday.

Mr Kamaev, responding to allegations of widespread doping among Russian athletes, urged commentators to distinguish between the laboratory and Russia's anti-doping agency, which he said was acting "in full compliance with the demands of the WADA codex".

His comments are the latest in a string of defensive remarks by Russian officials, following a hard-hitting report commissioned by WADA which alleged widespread doping by Russian athletes and official collusion in a cover-up.

Earlier, the Kremlin's spokesman, Mr Dmitry Peskov, described the allegations as "quite groundless".

The same laboratory processed tests for at least 20 other sports besides athletics, suggesting the alleged drug cheating could be widespread.

Russian Sports Minister Vitaly Mutko said the laboratory's head, Mr Grigory Rodchenkov, has resigned. "Rodchenkov, an experienced man, took the decision to resign to take all the negativity with him," Tass news agency quoted Mr Mutko as saying.

Mr Kamaev said there were question marks about the credibility of the sources used in the WADA report because they included sports people who had themselves failed doping tests.

"When the words of a sportsman who has broken the rules several times, and has already been disqualified, carry more weight than ours, then questions arise," he said.

He said Russia was working to clean up sport, emphasising the broadly efficient activity of the agency he heads.

"There are problems, but the objective facts, based on statistics, show that the Russian Anti-Doping Agency is quite effective," he said. "The agency takes the highest number of sanctions against transgressors in the world."