Russia on Tuesday extended by three months the detention of Evan Gershkovich, a Wall Street Journal journalist who was arrested in March for alleged spying, a charge he denies.

Gershkovich's pre-trial detention was due to expire on May 29 and the hearing in Moscow to extend the term was held behind closed doors.

"The court granted the investigator's request to extend the... detention until August 30," state-run agency RIA Novosti reported, citing the court press service.

Moscow says the sensitivity of the espionage charges means the trial must be held in private and court documents are not being made public.

Gershkovich, who previously worked for AFP, is the first foreign journalist arrested in Russia on spying charges since the collapse of the Soviet Union.

He has denied the accusations, and so have The Wall Street Journal and US officials.

Gershkovich is being held at Moscow's Lefortovo prison, which is famous for keeping detainees in near-total solitude.

US ambassador Lynne Tracy visited Gershkovich in mid-April and said the reporter was "in good health."

Russian authorities have since denied two consular visits to Gershkovich as part of a visa disagreement with the United States involving Russian journalists. (AFP)