The World Anti-Doping Agency (WADA) Monday gave Russia three weeks to explain "inconsistencies" in a cache of laboratory data handed over to investigators, raising the possibility of a fresh ban on the country in the build-up to the Tokyo Olympics.
Russia stands to be declared non-compliant by WADA if it fails to explain why evidence of some positive tests handed over by a whistleblower doesn't show up in data provided by Moscow's anti-doping laboratory in January.
If Russia challenges an eventual suspension by WADA the case will go to the Swiss-based Court of Arbitration for Sport, whose decision will be binding on sports bodies including the International Olympic Committee.
"Forensic experts have looked at what we got from whistleblowers, what we got from Russia and they noticed some inconsistencies," WADA director general Olivier Niggli told AFP after an executive committee meeting in Tokyo.
"Then they studied the differences and this came to a situation where there are some questions that need to be asked and answered."
WADA has previously warned that it would take the "most stringent sanctions" if any of the data was found to have been tampered with.
- 'Not soft on Russia' -
Russian sports minister Pavel Kolobkov said WADA and RUSADA, the Russian anti-doping agency, would look into 'inconsistences'.
"Digital experts from both sides, who are already in touch, will see what the discrepances are about and what they are connected to. As far as we are concerned, we continue to help in any way possible," he said in comments released by his ministry.
Russia handed over thousands of files from its Moscow anti-doping laboratory in January, fulfilling a key condition for its reinstatement by WADA last September.
RUSADA had been suspended for nearly three years over revelations of wrongdoing including a systematic conspiracy to switch tainted samples at the 2014 Sochi Winter Olympics.
During its suspension by WADA, Russia was allowed by the International Olympic Committee to take part in the 2016 Rio Olympics, but Russian competitors at the 2018 Pyeongchang Winter Games had to take part under a neutral flag.
However, athletics body the IAAF has maintained a ban on Russia since November 2015, although many of its athletes are given special dispensation to compete under a neutral banner.
The IAAF is due to re-examine the case for reintegrating Russian athletes ahead of the World Athletics Championships, which start in Doha this week.
Niggli said 47 doping cases which have already been identified from the computerised data were not affected by the latest revelation.
He defended WADA's handling of Russia, after criticism that the agency had softened its stance when it lifted its suspension a year ago.
"Potentially there is an issue, but we're dealing with it under a process which is clear to everybody, where the sanctions are defined and which might end up at CAS with a CAS decision that will bind every signatory to the court," Niggli said.
He added: "There's only so much WADA can do. We have no power to prevent any athletes to compete.
"People forget sometimes that apart from athletics, all Russian athletes are competing so if we had done nothing and waited and waited, the Russians would have continued competing as if nothing was happening.
"I don't think we've been soft on Russia but people expect us to have power that we simply don't have."