Argentina's security ministry says it has foiled a plot to use the Russian embassy's diplomatic courier service to smuggle cocaine to Europe.
Police seized nearly 400kg of the drug from diplomatic luggage in an embassy annex after being told about it by the Russian ambassador in December 2016.
They then mounted a sting operation, replacing the drugs with flour and adding tracking devices.
The operation has led to arrests in Argentina and Russia, officials say.
Argentine Security Minister Patricia Bullrich said the cocaine found in the Buenos Aires embassy was worth 50 million euros ($62m; £45m) and was of very high purity.
She said it was destined for Russia and probably also Germany.
In Russia, the foreign ministry spokeswoman said it had been established that the bags had belonged to a former member of the embassy's "technical staff" whose posting had already ended.
She said suspects from both countries had been arrested and are being investigated.
She dismissed as untrue media comments that the cargo would have been shipped using diplomatic channels.
The Argentine security minister, Patricia Bullrich, said the alleged mastermind of the smuggling operation was in Germany and she expected this person to be arrested by German police.
Two people have so far been arrested in Argentina and three in Russia.
Russian security services had helped in the investigation and one of those arrested in Argentina was a serving police officer in Buenos Aires, she said.