Russia protested to the United States on Saturday over a judge's refusal to grant bail to the former Kremlin aide, Pavel Borodin, who is being held in New York on Swiss money laundering charges.
Bail was denied in a court hearing on Friday by a US magistrate, who cited concerns that Borodin might try to seek refuge at the Russian embassy.
Borodin's lawyer, Barry Kingham, launched an immediate appeal against the ruling.
The Russian foreign ministry said in a statement that it rejected the refusal to grant bail, and suggested the Borodin case was becoming "more and more a political case than a legal one," adding that the affair could have a "negative affect on Russian-American relations."
Borodin was arrested in New York on a Swiss warrant in January, and is awaiting extradition to Switzerland on suspicion of money laundering.
The latest move follows a statement on Wednesday from Russia's prosecutor general, who confirmed he had found no evidence to support the allegations.
A spokesman for the Russian prosecutor's office said that checks carried out by Russia at the request of Switzerland had not revealed any financial irregularities relating to Borodin's time in office.
Russian prosecutors closed their investigation into the case last year, citing lack of evidence.
Swiss officials, however, say they have evidence that Borodin had access to Swiss bank accounts containing about $25 million (SFr42 million) and that the money has now disappeared.
They suspect that Borodin received the money in kickbacks from two Swiss-based firms, in return for lucrative contracts to renovate the Kremlin.
swissinfo with agencies