Russia on Monday accused a former US consulate employee of illegally passing data about the Ukraine conflict to American diplomats, drawing a protest from Washington which has called his activities routine.
The Russian Security Service (FSB) also announced it was seeking to question two US diplomats at the embassy in Moscow over the case, in a break from diplomatic tradition.
The announcement is the latest in a series of diplomatic spats building pressure on strained ties between Moscow and Washington, particularly over the conflict in Ukraine.
The FSB said it had charged Russian citizen Robert Shonov, who worked for more than 25 years for the US consulate in the Far Eastern city of Vladivostok, over "cooperation on a confidential basis with a foreign state".
The charge carries a maximum prison sentence of eight years.
Russian news agencies meanwhile released undated footage distributed by the FSB showing Shonov's detention on a snow-covered street. It also published images of Shonov testifying on camera.
Shonov left the consulate in 2021, when Moscow imposed restrictions on local staff working for foreign missions.
The US State Department says that Shonov since worked as a private contractor compiling press accounts from publicly accessible Russian media, "in strict compliance with Russia's laws and regulations".
"The allegations against Mr. Shonov are wholly without merit," State Department spokesman Matthew Miller said in response to the charges.
The action "only highlights the increasingly repressive actions the Russian government is taking against its own citizens," he said.
He also denounced Russian attempts to question US diplomats, pointing to obligations under the Vienna Convention.
"We strongly protest the Russian security services' attempts –- furthered by Russia's state-controlled media –- to intimidate and harass our employees," Miller said.
The FSB said Shonov had begun handing information to the American diplomats last September about the conflict and resulting mobilisation, an unpopular move in Russia that sparked an exodus of military-aged men.
It added that Shonov had been tasked with gauging protest sentiment ahead of presidential elections scheduled for next year.
The FSB said the two diplomats, identified as Jeffrey Silin and David Bernstein, worked in the political department in the US embassy in Moscow.
Russia foils latest Ukrainian drone attacks
Russian air defences downed Ukrainian drones over the Tula and Belgorod regions, Moscow's defence ministry said on Tuesday, without indicating if there had been damage or casualties.
Two drones "were destroyed" by air defences over the Tula region south of Moscow, Russia's defence ministry said in a statement on Telegram.
Another drone was "destroyed by air defence forces" over the Belgorod region, which borders Ukraine, at around 11 pm Moscow time (2000 GMT) on Monday, the ministry said in a separate statement.
The ministry did not say whether there had been damage or casualties as a result of either incident.
The capital and other Russian regions have been targeted by a barrage of Ukrainian drone attacks in recent days after Kyiv vowed earlier this summer to "return" the conflict to Russia.
The attacks have caused little serious damage.