US journalist Evan Gershkovich has been formally charged with spying in Russia, according to local media.
Mr Gershkovich, an experienced Russia reporter, was arrested last week in the city of Yekaterinburg while working for the Wall Street Journal (WSJ).
The media reports said he categorically rejected the accusations against him. The newspaper has demanded his immediate release.
Following his arrest, the Kremlin said he had been caught "red-handed".
Mr Gershkovich, 31, is well known among foreign correspondents in Moscow and BBC Russia Editor Steve Rosenberg describes him as an excellent reporter and a highly principled journalist.
The White House condemned his detention "in the strongest terms".
And on Friday in a rare joint statement, Senate Republican and Democratic leaders Mitch McConnell and Chuck Schumer strongly condemned his detention.
"Journalism is not a crime," they said. "We demand the baseless, fabricated charges against Mr Gershkovich be dropped and he be immediately released."
US officials say they have sought access to Mr Gershkovich but have not been able to visit him. However, the WSJ said its lawyers had been given access to him.
The Russian foreign ministry said the issue of consular access was being resolved, but added that the "fuss in the US about this case, which was aimed at pressurising the Russian authorities... was hopeless and senseless".
The WSJ said its reporter had dropped out of contact with his editors while working in Yekaterinburg, about 1,600km (1,000 miles) east of Moscow, on Wednesday afternoon.
US officials said Mr Gershkovich's driver had dropped him off at a restaurant and two hours later his phone had been turned off. The newspaper was unable to find him in the city.
Russia's FSB security service claimed that it had halted "illegal activities". The journalist had been detained while "acting on US instructions", it added, alleging that he had "collected information classified as a state secret about the activities of a Russian defence enterprise".
FSB agents took him to a Lefortovo district court in Moscow last Friday, where he was formally arrested and ordered to remain in detention until 29 May.
Espionage in Russia carries a maximum jail term of 20 years.
In his most recent WSJ piece, published last week, Evan Gershkovich reported on Russia's declining economy and how the Kremlin was having to deal with "ballooning military expenditures" while maintaining social spending.
Press freedom watchdog Reporters Without Borders said he had gone to Yekaterinburg to cover Russian mercenary group Wagner, which has taken part in some of the heaviest fighting in eastern Ukraine.
He has covered Russia for the Wall Street Journal for more than a year, having worked there previously for the AFP news agency and the Moscow Times. He began his career in the US.
