The United States renewed calls Tuesday for Russia to free Wall Street Journal reporter Evan Gershkovich after his detention was extended until August 30.

"We continue to call for his immediate release as well as for the immediate release of Paul Whelan," State Department spokesman Matthew Miller told reporters, referring to Gershhovich and detained former marine Whelan.

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.