US journalist appeals Russian detention: court

US journalist Evan Gershkovich, who was arrested in Russia last week on spying claims that he denies, is appealing his detention, a Moscow court said on Monday.

Gershkovich, 31, is believed to be the first foreign journalist arrested on spying allegations since the collapse of the Soviet Union.

His arrest has drawn outrage from the West and is seen as a serious escalation of Moscow's crackdown on media.

A spokeswoman for Moscow's Lefortovo district court said: "The date of the hearing on his detention appeal will be announced this week."

She added that the appeal was filed by the same lawyer that represented Gershkovich during his detention hearing.

Gershkovich, a Wall Street Journal reporter and former AFP journalist in Moscow, was remanded in custody until May 29.

He and the Wall Street Journal have denied the charges against him.

The Wall Street Journal said it "vehemently denies the allegations from the FSB and seeks the immediate release of our trusted and dedicated reporter."

More than three dozen news organisations have signed a letter to the Russian ambassador in the United States denouncing "unfounded espionage charges".

"Gershkovich's unwarranted and unjust arrest is a significant escalation in your government's anti-press actions," the letter released by the Committee to Protect Journalists said.

"Gershkovich is a journalist, not a spy, and should be released immediately and without conditions," it added.

State news agency TASS said the case against the journalist has been classified "top secret", a label which restricts information that can be published about it.