This undated photo provided by the Whelan family shows Paul Whelan in Iceland. Whelan, a former U.S. Marine arrested in Russia on espionage charges, was visiting Moscow over the holidays to attend a wedding when he suddenly disappeared, his brother said Tuesday. Credit: Courtesy of the Whelan Family via AP

MOSCOW — An American arrested in Russia has been formally charged with espionage, a Russian news agency reported Thursday, moving the case into Russia’s justice system and possibly deepening the diplomatic tensions with the United States.

The Interfax news agency report on Paul Whelan’s status could not be independently verified. “An indictment has been presented. Whelan dismisses it,” Interfax quoted a person familiar with the situation as saying.

Russian lawyer Vladimir Zherebenkov, who was appointed to represent Whelan, said the American will remain in custody in Moscow until at least Feb. 28. It was unclear whether court proceeding could begin before that date.

Whelan, 48, who was born in Canada and once served in the Marines, was detained last week by Russia’s domestic security services while he was in Moscow for what they described as a “spy mission.”

Whelan’s family denies the claims and have said they fear for his safety. It is believed Whelan also has Canadian citizenship. Ottawa confirmed a Canadian had been arrested in Moscow, but did not specifically name Whelan.

On Wednesday, the U.S. ambassador to Russia, Jon Huntsman, visited Whelan in a Moscow detention facility, marking the first contact U.S. officials have had with him since he was arrested at a hotel during a visit to attend a wedding in the Russian capital.

The visit came a few hours after Secretary of State Mike Pompeo said he expected officials from the U.S. Embassy in Moscow to be given access to Whelan within hours. Pompeo said they need to learn more about why Whelan was detained last Friday.

The timing of Whelan’s arrest — coming weeks after Russian gun rights activist Maria Butina pleaded guilty to Kremlin interference in the United States — has raised questions about a potential swap. The two countries do not have an extradition treaty.

The arrest of and guilty plea by Butina, 30, have become a sharp thorn in the side of U.S.-Russian relations. Butina is the first Russian national to be convicted of seeking to influence U.S. policy in the 2016 election campaign, and Moscow has gone to great lengths to paint her as a political prisoner.