(Updates to add name of RDIF's head, details)
By Maria Vasilyeva and Tom Balmforth
MOSCOW, Feb 21 (Reuters) - Russian state prosecutors on Thursday formally charged top U.S. investor Michael Calvey, the founder of the Baring Vostok private equity group, with fraud, hours before Washington accused Moscow of denying its diplomats access to him.
Calvey was first detained last week along with three other executives from his fund after investigators accused him and others of embezzling 2.5 billion roubles ($38.17 million).
Calvey denies the allegations against him, saying the case is being used to apply pressure on him in a business dispute over a Russian bank in which he is a shareholder.
The case has raised concerns among some foreign investors in Russia who are worried about the business climate. In a sign those concerns are being taken seriously in some circles, the head of Russia's RDIF sovereign wealth fund and its biggest state bank have spoken out in Calvey's favour.
RDIF head Kirill Dmitriev on Thursday requested via his lawyers that a Moscow court free Calvey from a pre-trial detention facility in Moscow and put him under house arrest instead.
Hours after Calvey was charged on Thursday, the U.S. embassy in Moscow said Russia was denying its diplomats access to him. It said it had asked to visit Calvey multiple times, but had not been granted access, something it said flouted consular rules between the two countries.
"We insist on access now," the embassy said in a statement.
Calvey is being held in a cell with seven other people and is experiencing back pain, Ivan Melnikov, a member of a public commission for the rights of inmates, told Reuters.
Melnikov, who visited Calvey in custody on Wednesday, said the U.S. national was sharing a cell with people being held over non-economic crimes in contravention of prison regulations.
Russian Foreign Minister Sergei Lavrov said on Thursday that Calvey's case was in the political spotlight.
"The question lies at the centre of a very lively political discussion. And I think that all the circumstances of the case will be taken into account when the legal hearings take place soon," Lavrov was quoted as saying by Russian news agencies.
"As far as I understand he was not a very visible person in the public realm. But of course I've heard what people whom I deeply respect have said about him, and I think many people have heard this," Lavrov said.
Calvey is due to appeal his arrest next week. ($1 = 65.4925 roubles) (Additional reporting by Andrey Kuzmin; writing by Tom Balmforth; editing by Andrew Osborn)