MOSCOW (Reuters) - A prominent U.S. investor being held in Russia on embezzlement charges has asked U.S. diplomats not to defend him because he is worried about his case becoming politicized, a prison monitor who visited him in custody said on Tuesday.

The detention last month of Michael Calvey, founder of the Baring Vostok private equity group, has strained U.S.-Russia ties and stoked talk of a possible U.S. boycott of an annual economic forum in June attended by President Vladimir Putin.

Calvey was detained along with three other executives from Baring Vostok after investigators accused them of stealing 2.5 billion roubles ($38.09 million), a charge he denies.

Ivan Melnikov, a member of a public body that monitors prisons, visited Calvey in custody on Monday and said the investor told him a U.S. diplomat had visited him and that Calvey had asked them not to play up his case publicly.

“He has asked (the U.S. diplomat) not to get actively involved in his defense because he is worried about the case becoming politicized,” Melnikov said.

Ties between Russia and the United States are at post-Cold War lows, strained by issues such as Moscow’s annexation of Crimea, the war in Syria, and espionage cases.

Calvey’s arrest has rattled some foreign investors who are worried about the business climate and some prominent Russian businessmen have called for him to be released from pre-trial detention on bail, saying his treatment has been too harsh.

The Kremlin has said Vladimir Putin does not plan to intervene in the case and that investigators should be allowed to do their job.

Melnikov said the U.S. investor was having problems receiving correspondence, but was in satisfactory condition.

“Calvey said he plans to continue with his projects in Russia when he gets out of pre-trial detention,” Melnikov said.

“He hopes they’ll get to the bottom of this case soon.”