Wall Street Journal reporter Evan Gershkovich has appealed against a three-month extension of his detention on espionage charges, a Russian court data website indicated on Friday.
Mr Gershkovich, 31, a US citizen, was arrested in March while on a reporting trip to Russia.
He, his employer and the US government have denied the charges. A Moscow court on Tuesday extended his detention until August 30.
According to the court website, the appeal was filed at the Lefortovo district court on Thursday. There was no information whether a date has been set for a hearing.
Mr Gerhskovich’s case has been wrapped in secrecy. Russian authorities have not detailed what — if any — evidence they have gathered to support the espionage charges.
Various legal proceedings in the case have been closed to the media. State news agency Tass said Tuesday’s hearing was held behind closed doors because the reporter was accused of possession of “secret materials”.
The Wall Street Journal reported on Tuesday night that Mr Gershkovich’s parents, Soviet emigres living in New Jersey, were visiting Moscow and saw their son during the short hearing.
“I don’t know how to describe this happiness and this sadness at the same time,” the newspaper quoted the reporter’s mother, Ella Milman, as saying. She said Mr Gershkovich looked relaxed and well, and that they communicated through smiles.
The US State Department said at least one US Embassy official also attended the hearing.
Mr Gershkovich’s arrest has drawn outrage in the West.
The US government declared Mr Gershkovich to be wrongfully detained and demanded his immediate release. He is being held in Moscow’s Lefortovo prison.
US Embassy officials were allowed to visit Mr Gershkovich once in prison since his arrest in Yekaterinburg on March 29, but Russian authorities have denied two more recent requests to see him.