A Russian court on Thursday rejected an appeal by a U.S. private equity founder and ordered him to remain behind bars for at least the next six weeks in connection to an international fraud case.

Michael Calvey, one of the most prominent American business executives in Russia, was among five people detained in relation to the alleged embezzlement of $37.5 million earlier this month.

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The Moscow City Court's decision to keep Calvey detained until April 13 comes despite multiple statements on Calvey's behalf from prominent members of the Russian investment community.

“This is a purely commercial dispute and should be decided in a civil court, not a criminal one,” Calvey, 51, told the court before the ruling.

Calvey, a U.S. citizen who grew up in Oklahoma and co-founded private equity firm Baring Vostok in 1994, was arrested on suspicion they embezzled the money from Vostochny Bank, in which Baring Vostok has a controlling stake. The company claims it has invested $2.8 billion from North American pension funds and others into companies in Russia and across the former Soviet Union.

U.S. Embassy spokeswoman Andrea Kalan tweeted Tuesday that "an Embassy representative met with Mr. Calvey to express our support for his well-being and offer the Embassy's assistance."

Kalan added: "We take seriously our right to visit detained U.S. citizens regularly and ensure they receive humane treatment and access to medical care."

The embassy had previously complained that Russia was not allowing access to Calvey, who is spending his 14th day in detention over the alleged fraud. Four days is the typical limit to grant diplomatic access to U.S. detainees in Russia.

The punishment for fraud in Russia can be up to 10 years in prison.

US BUSINESSMAN ARRESTED IN RUSSIA ON FRAUD CHARGES, ACCUSED OF EMBEZZLING $37M

Calvey is the third Westerner to be targeted by Moscow in the past few months. In early February, a Russian regional court sentenced a Danish Jehovah's Witness to six years in prison after being found guilty of extremism.

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In December, former U.S. Marine Paul Whelan was arrested and charged with espionage.

The Associated Press contributed to this report.