"In light of V.A. Bout's transfer to a special block of the Marion prison, Illinois, with additional restrictions, we have sent corresponding notes of protests to the State Department. We have urged the US authorities to cancel the unjustifiably severe measures," the Embassy said in a statement.
Russian diplomats will soon visit Bout, according to the statement.
Viktor Bout was arrested in Thailand per the US request in 2008 and was charged with conspiracy to commit murder of US citizens as well as illegally selling portable surface-to-air missiles.