The United States ambassador in Moscow accused Russian authorities on Monday of making a mockery of justice after he was turned away for a second time from what he called a "secret" trial behind closed doors of an ex-US Marine charged with espionage.

Russia last week began the trial of US national Paul Whelan on charges of spying after his arrest by Russian security agents in a hotel room sting operation in December 2018. He denies the charge.

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The case, as well as that of Michael Calvey, a US investor charged with embezzlement in Russia, has complicated already strained relations between Moscow and Washington.

Whelan, who also holds British, Canadian and Irish citizenship, faces up to 20 years if found guilty.

The Moscow court had said the trial would involve classified information and would therefore not be open to the public, but US Ambassador John Sullivan said he had tried to attend the hearing in Moscow on Monday and had been turned away.

"The fact that it is a closed hearing, that it is a secret trial - Paul hasn't seen the evidence against him - it makes a mockery of justice," Sullivan said in remarks carried on the US embassy's website.

He urged Russia's authorities to ensure Whelan would receive a fair and impartial trial, to grant him a phone call to his family and to allow him medical treatment.

"He hasn't been allowed to make a phone call, to speak to anyone in his family in 16 months," Sullivan said.

It makes a mockery of justice John Sullivan, US ambassador to Moscow

Whelan, who turned 50 in custody last month, has used his appearances at hearings since his arrest to allege he has been ill-treated by prison guards and also denied medical attention.

Russian authorities have dismissed his remarks and accused Whelan of faking health problems in custody to draw attention to his case.

The trial's preliminary hearings began on March 23 even as many court hearings have been put on hold because of the coronavirus lockdown.

Moscow says Whelan was caught red-handed with classified information, but his lawyer has said he was set up and thought he was receiving holiday photos from a Russian acquaintance.

"Paul said that he believes he was framed and that [the case] was a provocation against him," Olga Karlova, one of his legal team, told CBS News last week. He appeared to be in good spirits, she added.

Russian security agents detained Whelan in a hotel room in December 2018, seizing a USB drive allegedly containing top-secret information. Whelan says the device was planted on him while in the Russian capital attending a wedding.

When his detention was extended in May last year, Whelan told reporters he was a "victim of political kidnap and ransom".