Evan Geshkovich's detainment marks a serious escalation in Kremlin's efforts to silence perceived critics, a crackdown that gained momentum following Russia's attacks on Ukraine last year.
A US reporter for The Wall Street Journal newspaper has been detained in Russia on charges of spying for Washington, Russian news agencies reported, citing the FSB security services.
The FSB security services said on Thursday they had "halted the illegal activities of US citizen Evan Geshkovich," who was "suspected of spying in the interests of the American government".
Their statement confirmed that Geshkovich, 31, was working with press accreditation issued by the Russian foreign ministry. But said he had been detained for gathering information "on an enterprise of the Russian military-industrial complex".
"The foreigner was detained in Yekaterinburg while attempting to obtain classified information," the FSB said, referring to a city in central Russia 1,800 kilometres east of Moscow.
Following the news, The Wall Street Journal said it "vehemently denies" Russian allegations about Gershkovich.
Meanwhile, a US diplomatic source said the embassy had not been informed about the incident and was seeking information from the Russian authorities about the case.
Gershkovich's arrest comes as Western journalists in Russia face increasing restrictions.
Staff of Western media outlets often report being tailed, particularly during trips outside of major urban hubs of Moscow and Saint Petersburg.
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Who is Evan Geshkovich?
Other foreign journalists covering Russia expressed support for Gershkovich online, saying he is a professional journalist, not a spy.
Andrei Soldatov, an author and expert in Russia's security agencies who is outside the country, said on social media: "Evan Gershkovich is a very good and brave journalist, not a spy."
Soldatov said Gershkovich's detention "is a frontal attack on all foreign correspondents who still work in Russia. And it means that the FSB is off the leash."
Before joining The Wall Street Journal, 31-year-old Gershkovich worked for AFP news agency in Moscow. He was also previously a reporter for The Moscow Times, an English-language news website.
His family immigrated to the United States from Russia when he was a child.
He is now among several US citizens are currently in detention in Russia and both Washington and Moscow have accused the other of carrying out politically-motivated arrests.
There have also been several high-profile prisoner exchanges between Moscow and Washington over the past year.
In December, Moscow freed US basketball star Brittney Griner — arrested for bringing cannabis oil into the country — in exchange for Russian arms dealer Viktor Bout.
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