Russia on Thursday extended by three months the detention of Wall Street Journal correspondent Evan Gershkovich, defying pleas for the release of the reporter arrested on the job in March.

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

He was the first journalist arrested by Moscow on allegations of spying -- which he and his employer strongly deny -- since the Cold War, sending a chill through media circles.

"The time of detention has been extended by three months," a spokesperson for Moscow's Lefortovsky court said.

The new end-date for the detention is November 30.

AFP was denied access into the courtroom for the hearing, which was held behind closed doors.

But AFP reporters saw the 31-year-old, handcuffed and wearing a checkered shirt and jeans, being escorted into the court by masked men.

Responding to the decision, the State Department renewed calls on Russia to release "immediately" both Gershkovich and another detained American, former Marine Paul Whelan.

"The claims against Evan Gershkovich are baseless," a State Department spokesperson said.

The Wall Street Journal said it was "deeply disappointed" that Gershkovich will remain "arbitrarily and wrongfully detained for doing his job as a journalist".

"The baseless accusations against him are categorically false, and we continue to push for his immediate release," the New York-based newspaper said in a statement.

"Journalism is not a crime."

US President Joe Biden has previously called for the reporter's release.

In Germany, Foreign Minister Annalena Baerbock also said Thursday that Gershkovich "must be released".

The Moscow court's decision "shows what this brutal force at home means -- that there are no fair trials" in Russia, she said.

Arrested while reporting

Gershkovich was arrested during a reporting trip in the Urals city of Yekaterinburg on March 29.

Russia has not provided public evidence of the accusations and the legal proceedings have been classified as secret.

Gershkovich, who previously reported for AFP, has written extensively about how ordinary Russians have experienced the Ukraine offensive, including by speaking to the families of dead soldiers.

At the time of his arrest, he was believed to be working on a story about the Wagner private military company, which has been fighting in Ukraine.

Wagner's boss, Yevgeny Prigozhin, later staged a brief revolt inside Russia. He was presumed dead on Wednesday after Russian officials said he was on a plane that crashed, with all passengers killed.

Since his arrest Gershkovich has been held in Moscow's notorious Lefortovo prison, famous for keeping inmates in near-total isolation.

The US ambassador to Russia, Lynne Tracy, was able to see Gershkovich last week in her third visit since his detention.

The ambassador said the reporter appeared to be in good health and spirits despite the circumstances.

Many Western journalists left Russia after the Kremlin sent troops to Ukraine, when conditions for reporting became more difficult and Moscow made it harder for journalists to obtain accreditation.

Several US citizens have been handed heavy sentences in Russia in recent years.

Whelan, the other detained American, was able to speak by telephone last week with US Secretary of State Antony Blinken.

The United States has frozen most high-level contact with Russia but has arranged two swaps of prisoners, including one deal that freed basketball star Brittney Griner, arrested over cannabis detected in vape cartridges.

© Agence France-Presse