Alsu Kurmasheva, an editor at US government-funded Radio Free Europe/Radio Liberty (RFE/RL), was detained last week.

Her employer said the court ordered she remain in custody until 5 December, rejecting her lawyer's request for other measures than imprisonment.

Ms Kurmasheva had her passports confiscated when she was stopped at Kazan International Airport in June.

She was fined for failing to register her US passport and was unable to leave Russia.

Her employer said the foreign agent charge was brought against her on Wednesday as she awaited the return of her documents.

Russian state-run news website Tatar-Inform said Ms Kurmasheva was collecting information on Russian military activities.

The news agency had showed footage of men in balaclavas taking Ms Kurmasheva into an administrative building.

She could face up to five years in prison should she be convicted.

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The charges against Ms Kurmasheva have been deemed "spurious" by The Committee to Protect Journalists saying her detention "is yet more proof that Russia is determined to stifle independent reporting".

RFE/RL was informed by Russia in 2017 to register as a foreign agent but it challenged Moscow's use of foreign agent laws in the European Court of Human Rights. The organisation has been heavily fined by Russia.

Ms Kurmasheva lives in Prague and is an editor with RFE/RL's Tatar-Bashkir service, reporting on ethnic minority communities in Tatarstan and Bashkortostan in Russia.

She is the second US journalist to be detained in Russia this year, after The Wall Street Journal's Evan Gershkovich was arrested for alleged spying in March.

Analysts believe Russia is detaining US citizens to use as bargaining chips amid tensions over the Ukraine war.

Basketball star Brittney Griner was held for many months before being swapped for a Russian arms dealer jailed in the US.

Evan Gershkovich, meanwhile, has appeared in court a number of times.

He and the WSJ deny the spying allegations and the US government has said he's being wrongfully detained.

Detailed evidence has yet to be provided by Russian authorities.