Radio Free Europe/Radio Liberty has called the move one of "desperate attempts" at censorship after bailiffs visited its Moscow office. Meanwhile, Russia has declared the US to be an "unfriendly state."
Russia sent bailiffs to the Moscow bureau of US-funded Radio Free Europe/Radio Liberty (RFE/RL) on Friday over unpaid fines.
RFE/RL said the move was an escalation in the Russian government's campaign to push them out of the country.
"RFE/RL will continue to fight these desperate attempts by the Kremlin to censor uncomfortable truths," said RFE/RL President Jamie Fly in a statement.
US State Department spokeswoman Jalina Porter told reporters Washington was "deeply troubled" at the move. It represented Russia's "latest attempt to suppress independent media and deny the Russian people access to objective news," she said.
RFE/RL was designated as a "foreign agent" in Russia in 2017, a claim that the news outlet denies. Groups identified as foreign agents in Russia must disclose their sources of funding and media outlets under this tag must publish a disclaimer saying their publications are "fulfilling the function of a foreign agent." If they refuse, they face fines.
Groups that are considered foreign agents face difficulties in funding and a stigma that can make it difficult to continue their work.
Russian media watchdog Roskomnadzor filed 520 violation cases against RFE/RL, which could entail fines up to $2.4 million (€1.98 million).
Russia also designated the online news site VTimes a "foreign agent" on Friday.
The Justice Ministry said the outlet had been added to the list of "foreign agents" because it is registered in the Netherlands.
VTimes did not immediately comment on the issue.
Meanwhile, Russian Prime Minister Mikhail Mishustin formally designated the US and Czech Republic as "unfriendly states" on Friday.
In a decree, he said both nations "have carried out unfriendly actions" against Russia, Russian nationals or entities. Kremlin spokesman Dmitry Peskov said Russia was still ready for dialogue, according to state news agency TASS.
Moscow added that the Czech Republic will be allowed to employ no more than 19 Russian nationals, and the US Embassy will not be allowed any.
The Czech Republic said the action would "escalate relations" between Moscow and Prague, the EU and its allies.
Russia-US relations have rapidly fallen apart after US President Joe Biden increased pressure on the Kremlin since taking office in January. The US announced sanctions on Russia in April and expelled 10 Russian diplomats in retaliation for what it says was interference in US elections, a cyberattack and other hostile activity.
Russia responded by expelling 10 US diplomats and banning top US officials from entering the country.
kbd/rt (AP, AFP, Reuters)