MOSCOW (Reuters) - A former U.S. Marine detained in Russia has been charged with espionage, the Interfax news agency said on Thursday, meaning he could face up to 20 years in prison if convicted.

Russia’s FSB state security service detained Paul Whelan, 48, in Moscow last Friday on suspicion of spying, in a case which threatens to aggravate diplomatic tensions with the United States.

The FSB opened a criminal case against Whelan but gave no details of his alleged espionage activities.

Interfax said the Russian authorities had brought formal charges against Whelan on Thursday, citing what it described as an informed source. Reuters could not independently verify the Interfax report.

Whelan’s lawyer, Vladimir Zherebenkov, declined to comment on the charges but told Reuters that under the terms of the arrest order, Whelan was expected to remain in custody in Moscow until at least Feb. 28.

“I consider his detention and arrest baseless. It’s based on investigators’ supposition that he will hinder the investigation process. We are asking for bail instead,” Zherebenkov told Reuters by telephone.

U.S. Secretary of State Mike Pompeo said on Wednesday the United States had asked Russia to explain Whelan’s arrest and would demand his immediate return if it determines his detention is inappropriate.

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Whelan’s family has said that he was visiting Moscow for the wedding of a retired Marine and is innocent of the espionage charges against him.

Under Russian law, espionage can carry sentences of between 10 and 20 years in prison.

“FEELING POSITIVE”

Zherebenkov said Whelan, who is being held at the Lefortovo detention facility in Moscow, was “feeling positive” and that a translator was explaining the legal procedures to him.

“All his rights and all international principles are being fully followed,” Zherebenkov said.

Russia’s relations with the United States plummeted when Moscow annexed the Crimean Peninsula from Ukraine in 2014. Washington and Western allies imposed sanctions on Russian officials, companies and banks.

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U.S. President Donald Trump’s relations with his Russian counterpart Vladimir Putin have been under a microscope because of U.S. Special Counsel Robert Mueller’s investigation into suspected Russian meddling in the 2016 U.S. presidential election and possible collusion with the Trump campaign.

Moscow has denied interfering in the election. Trump has said there was no collusion and says Mueller’s probe is a witch hunt.

Daniel Hoffman, a former CIA Moscow station chief, said it was “possible, even likely” that Putin had ordered Whelan’s arrest to set up an exchange for Maria Butina, a Russian citizen who pleaded guilty on Dec. 13 to acting as an agent to influence conservative groups in the United States.

Russia says Butina was forced to make a false confession about being a Russian agent.

The Kremlin did not immediately respond to a request for comment on Whelan’s detention.

Putin told Trump in a letter on Sunday that Moscow was ready for dialogue on a “wide-ranging agenda,” the Kremlin said, after a series of attempts to schedule a meeting between the leaders.

At the end of November, Trump abruptly canceled a planned meeting with Putin on the sidelines of a G20 summit in Argentina, citing tensions about Russian forces opening fire on Ukrainian navy boats and then seizing them.