A Russian court on Monday extended until June 5 the pre-trial detention of US-Russian journalist Alsu Kurmasheva, who faces 15 years in prison on charges of spreading "false information," according to her employer.

In court in the western city of Kazan on Monday, Kurmasheva smiled but complained about the poor state of the cell where she was being held, an AFP reporter said.

A journalist at the US-funded Radio Free Europe/Radio Liberty (RFE/RL), she was arrested last year for failing to register as a "foreign agent".

RFE/RL says she was subsequently charged with spreading false information under new censorship laws following Russia's military offensive on Ukraine in 2022.

In 2022, Kurmasheva edited a book titled, "Saying No to War" -- a collection of interviews and stories from Russians opposed to Moscow's campaign against Ukraine.

RFE/RL on Monday called her imprisonment "outrageous" and said she had been locked up "simply because she holds an American passport."

"The charges against Alsu are baseless. It's not a legal process, it's a political ploy, and Alsu and her family are unjustifiably paying a terrible price," RFE/RL head Stephen Capus said.

"Russia must end this sham and immediately release Alsu without condition," he added.

Kurmasheva, who lives in Prague with her husband and two children, had her US and Russian passports confiscated last June after travelling to Russia for a family emergency.

She was then arrested for failing to register as a "foreign agent" in October while awaiting the return of her passports.

That charge carries up to five years in prison while spreading "false information" has a maximum sentence of 15 years.

Rights groups have accused Russia of using oppressive legislation to target regime critics and independent journalists.

Kurmasheva is the second US journalist to be arrested in Russia since the start of Moscow's military offensive in Ukraine.

Wall Street Journal reporter Evan Gershkovich has spent more than a year in jail in Moscow on espionage charges that carry a maximum sentence of 20 years in prison.

He too has denied the charges.

Kurmasheva's lawyers had called for her to be released from prison and put under house arrest pending the trial.

The hearing did not concern the substance of the case.

The US State Department said last year that Kurmasheva's arrest "appears to be another case of the Russian government harassing US citizens".

Washington has accused Moscow of arresting US citizens without evidence to swap for the release of Russians jailed abroad.