A Russian athlete at the Pyeongchang Winter Olympics has committed a "possible violation of anti-doping rules", the team's spokesman told Russian media on Sunday.
"Today the headquarters of our delegation received an official notice from the IOC regarding a possible violation of anti-doping rules," Konstantin Vybornov, spokesman for the Olympic Athletes from Russia, was quoted as saying.
"We are not naming the athlete or the discipline until the B-sample results, which are expected within approximately 24 hours."
Vybornov was not immediately available to confirm the statement, which was carried by several Russian media including the TASS news agency.
The Court of Arbitration for Sport, which is handling doping cases in Pyeongchang, did not respond to a request for confirmation of the case.
Russia's Sport Express said the case involved meldonium, which was added to the World Anti-Doping Agency's list of banned substances in 2016.
Russia are banned from the Olympics over systemic doping, but 168 Russian athletes deemed clean are competing in Pyeongchang under the neutral banner of Olympic Athletes from Russia.
A doping case would be deeply embarrassing and could affect deliberations on whether to lift Russia's ban in time for the Pyeongchang closing ceremony.
The International Olympic Committee suspended Russia in December after revelations of a widespread and highly orchestrated doping conspiracy, which first emerged before the Rio 2016 Summer Games.
Investigations found that the doping plot, straddling several years, culminated when Russia hosted the Sochi 2014 Winter Olympics, where they topped the medals table.
Russian athletes in Pyeongchang are under strict instructions to honour the "letter and spirit" of guidelines governing their participation, including not waving the Russian flag or wearing its colours.
An IOC implementation panel is due to consider how well the Russian team has observed the guidelines before making a recommendation on whether to lift the ban.
Russian tennis player Maria Sharapova, a five-time Grand Slam winner and former world number one, was banned for 15 months after testing positive for meldonium in early 2016.
Russian short track speed skater Semen Elistratov, who won bronze in the men's 1,500m in Pyeongchang, served a brief ban for the same substance in 2016.
Richard Budgett, medical director for the International Olympic Committee, said it seemed that Elistratov took the drug before it was banned at the start of 2016.
"Because of the way it's metabolised, it can stay in the system for many, many months, even nine months after it had been taken," Budgett told journalists last week.
He added: "We have to be realistic — meldonium was used in a widespread way throughout Eastern Europe and Russia and was considered to be a tonic, a type of cardiac stimulant that was not prohibited.
"But of course, quite rightly it was then prohibited and because it was such widespread use there were a large number of cases."