US reporter Evan Gershkovich has appealed against a Russian court's decision to extend his pre-trial detention by three months, according to documents published by a Moscow court.

Gershkovich was detained in March during a reporting trip to the Urals and accused of spying -- charges that he, the US government and his employer the Wall Street Journal vehemently deny.

His pre-trial custody was due to expire on August 30, but a judge ruled Thursday it would be extended to November 30, a decision criticised by the WSJ and US State Department.

The Moscow City Court website showed this additional three-month extension had been "appealed" by his defence.

Gershkovich, unlike many Western reporters, had continued to report from Russia during Moscow's offensive in Ukraine.

His case marks the first time a Western journalist has been arrested on espionage charges in Russia since the Soviet era.

Since launching full-scale hostilities against Ukraine last year, Russia has made it more difficult for journalists from the West to obtain accreditation and work in the country.

Several US citizens have been handed heavy sentences in Russia in recent years. Washington accuses Moscow of using them as bargaining chips to obtain the release of Russians held in the US.