MOSCOW (Reuters) - A former U.S. Marine held in Russia on suspicion of spying is being illegally isolated in a Moscow pre-trial detention center and prevented from communicating with visitors, Russian rights activists and U.S. diplomats said.

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Paul Whelan, who holds U.S., British, Canadian and Irish passports, was detained in a Moscow hotel room on Dec. 28 and accused of espionage after being handed a flash drive his lawyer said Whelan thought contained holiday photos but which actually held classified information.

Whelan, 49, who faces up to 20 years in prison if convicted, believes he was framed and that the case against him is politically motivated revenge for U.S. sanctions imposed on Russia.

He used a court hearing last month to say that his life had been threatened by a Russian investigator and that he was being harassed in custody in an effort to force him to talk.

Andrea Kalan, a spokeswoman for the U.S. embassy in Moscow, said on Wednesday that officials at the Moscow prison where he is being held ahead of his trial were blocking human rights activists from communicating with Whelan.

“What are investigators hiding?” Kalan, who has previously complained about a lack of evidence in the case, wrote on Twitter.

She referred to an article by a member of Russia’s presidential human rights council who has visited Whelan in jail at least three times.

The article by Eva Merkacheva, a journalist for the daily Moskovsky Komsomolets newspaper and member of the rights council, said that guards at Moscow’s Lefortovo prison had prevented her and officials who monitor prison conditions from communicating with Whelan in English, the only language the former marine speaks.

“They (guards) consider that Paul might pass us a secret code,” Merkacheva wrote, saying Whelan’s cell door had been slammed in their face after he had addressed them in English.

“The scene was not simply ugly but harrowing,” she said. “We didn’t find out what it was he wanted to complain about.”

Merkacheva said the authorities had not allowed the use of an interpreter either, and that one of the few things they had managed to hear from Whelan was that the investigator had for two months not allowed him access to any dictionaries.

“Our view that something strange is going on with Paul Whelan has only been reinforced,” she said, adding that there was nothing in Russian law to justify the prison’s decision to ban the use of English.

She said the rights council, which advises President Vladimir Putin, would write to the prosecutor general’s office to challenge the legality of the prison’s language ban.