The FSB said they had "halted the illegal activities of US citizen Evan Gershkovich," saying the Wall Street Journal reporter was "suspected of spying in the interests of the American government".
The Wall Street Journal said it was "deeply concerned for the safety of Mr. Gershkovich" and the international media watchdog Reporters Without Borders (RSF) said it was "alarmed by what looks like retaliation".
RSF said Gershkovich "was investigating on the military company Wagner" -- a mercenary group playing a prominent role in Russia's campaign in Ukraine.
The FSB confirmed that Gershkovich, 31, was working with press accreditation issued by the Russian foreign ministry.
It said he had been detained for gathering information on Russia's "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 kilometers (1,100 miles) east of Moscow.
Russia's foreign ministry spokeswoman Maria Zakharova alleged that Gershkovich had been "caught red handed".
Before joining The Wall Street Journal Gershkovich worked for AFP in Moscow.
A fluent Russian speaker, he was previously a reporter based in the Russian capital for The Moscow Times, an English-language news website.
His family immigrated to the United States from Russia when he was a child.
"The problem is... the fact that the way the FSB interprets espionage today means that anyone who is simply interested in military affairs can be imprisoned for 20 years," Russian political analyst Tatiana Stanovaya said on social media in response to the detention.
Several US citizens are currently in detention in Russia and both Washington and Moscow have accused the other of carrying out politically-motivated arrests.
The FSB in January opened a criminal case against a US citizen it said was suspected of espionage but did not name the individual.
Paul Whelan, a former US Marine, was arrested in Russia in 2018 and handed a 16-year sentence on espionage charges. He is detained in a penal colony south of Moscow.
The US says he was a private citizen visiting Moscow on personal business and has demanded his release.
There have 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.
Russian authorities have also used espionage charges against Russian journalists.
Last year, Russia jailed a former defense reporter, Ivan Safronov, for 22 years on treason charges.
Safronov worked for business newspaper Kommersant and was one of Russia's most prominent journalists covering defense.
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.
Many Russians fear speaking to foreign media, due to strict censorship laws adopted in the wake of the Ukraine offensive.
© 2023 AFP