Former U.S. Marine Paul Whelan, who was detained and accused of espionage, speaks inside a defendants' cage during a court hearing to consider an appeal to extend his detention in Moscow, Russia June 20, 2019. REUTERS

Moscow -- The U.S. Embassy in Moscow says the condition of an American man held on spying charges in Russia has worsened. The Embassy said on Twitter Monday that Russian authorities had rejected a request to send a doctor to examine Paul Whelan. It said Whelan has received basic medical assistance, but that his condition has deteriorated.

Whelan, who also holds British, Irish and Canadian citizenship, was arrested in a hotel room in Moscow in December and charged with espionage, which carries up to 20 years in prison.

The former U.S. Marine has publicly complained of poor prison conditions, including alleged abuse, and said his life is in danger.

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"We asked in April for an outside exam at no cost to the Russian govt. Request denied, yet Paul's condition worsens," U.S. Embassy spokesperson Andrea Kalan said Monday on Twitter, adding that the, "welfare of U.S. citizens abroad is our highest priority."

#PaulWhelan gets basic medical care at Lefortovo, but his condition needs more than the prison can provide. We asked in April for an outside exam at no cost to the Russian govt. Request denied, yet Paul’s condition worsens. Welfare of U.S. citizens abroad is our highest priority. https://t.co/AewxpDH2kh — Andrea Kalan (@USEmbRuPress) July 1, 2019

Russia's Deputy Foreign Minister Sergei Ryabkov on Monday denied plans to exchange Whelan for Russians in U.S. custody.

Last month during a court hearing at which the judge granted an extension of Whelan's detention until the end of August, the prisoner appealed directly to President Trump to help get him out of jail.

Whelan, who also holds British, Irish and Canadian citizenship, called his arrest an "absurd political kidnap" at the hearing. He urged the governments of the U.K., Ireland and the U.S. to help him.

"I'm asking government leaders, authorities -- Ottawa, Dublin, London and Washington -- to help and express public support," Whelan was quoted as saying by Russia's state-run RIA Novosti news agency.

Addressing Mr. Trump, Whelan read from a statement: "Mr. President, we cannot keep America great unless we aggressively protect American citizens wherever they are in the world."

Whelan's Russian lawyer Vladimir Zherebenkov told CBS News' Elizabeth Palmer outside the court in February when Whelan was arraigned that his client was found at his hotel with a flash drive containing Russian "state secrets." Zherebenkov has maintained that his client was handed the flash drive and didn't know that it contained any classified information.

The lawyer has not explained in detail how Whelan came to be in possession of the information on the drive, but said his client wasn't aware that the Russian government considered the material sensitive. Zherebenkov added that the prosecution did not "have strong evidence to back up the charges" against Whelan.

U.S. intelligence and State Department sources have told CBS News they're confident Whelan is not a spy.

At the hearing in May during which his detention was extended, Whelan complained to reporters of alleged abuse in custody.

"I have been threatened. There are abuses and harassment that I am constantly subject to. There is a case for isolation. I have not had a shower in two weeks, I can't use a barber I have to cut my own hair. I can't have medical treatment, I can't have dental treatment… They are trying to run me down so that I will talk to them," he said.