The comments from the U.S. Embassy in Moscow were the sharpest yet from Washington in the case of Paul Whelan, who was detained in a Moscow hotel room on Dec. 28 and accused of espionage, a charge he denies.
Whelan, 49, holds U.S., British, Canadian and Irish passports, and if found guilty could be imprisoned for up to 20 years.
His case has dragged already dire U.S.-Russia relations lower, as has that of another American, prominent investor Michael Calvey, who is under house arrest while being investigated on embezzlement charges.
U.S. Ambassador Jon Huntsman visited Whelan in a Moscow jail on Tuesday. Following this Andrea Kalan, a spokeswoman for the embassy, said the situation surrounding Whelan was beyond a joke.
"Today marks 124 days in detention for #PaulWhelan," Kalan wrote on social media.
"Why haven’t Russian officials provided proof? Perhaps it’s lost along with Paul’s mail. Complete lack of evidence + Paul's isolation = greater likelihood officials will try to get a forced false “confession.” Stop playing games," she wrote.
Kalan complained that Huntsman's visit was "a joke" since under the rules of Moscow's Lefortovo jail he was unable to read aloud letters from his family, unable to ask him to sign a power of attorney, and banned from "talking about anything that actually matters."
Russia's Federal Security Service (FSB) detained Whelan after an acquaintance handed him a flash drive containing classified information. Whelan's lawyer says his client thinks he was set up by the acquaintance and the FSB.
Whelan thought the flash drive contained holiday photos, the lawyer has said.
Russia's Ministry of Foreign Affairs has previously dismissed U.S. criticism of Moscow's handling of the case.
Whelan is due to be kept in pre-trial detention until May 28 while investigators continue to look into his case.A Russian court this month extended until July 14 the house arrest of Calvey. The investor denies the charges against him.