The trial of Paul Whelan, 50, is continuing behind closed doors in a Moscow court despite the coronavirus pandemic and diplomatic protests.

Whelan's lawyer Vladimir Zherebenkov told news agency Interfax that the witness had testified against Whelan.

The lawyer did not provide further details of the session but said the defence would dispute the account offered by the witness.

"We believe there are contradictions in his testimony," Zherebenkov said.

US ambassador John Sullivan was not allowed to attend Monday's hearing, the embassy said in a statement.

The ambassador criticised Russian authorities for not allowing Whelan to contact family and refusing to provide medical care.

"He needs medical treatment for a medical issue that could be treated very simply, but it is a serious issue and the Russian government has refused," Sullivan said in the statement. "That's intolerable, and it is unacceptable."

Russian authorities said journalists and embassy employees are not allowed to attend the hearings due to the coronavirus epidemic.

Whelan, who also has Irish, Canadian and British citizenship, was arrested in December 2018 for allegedly receiving state secrets. He risks up to 20 years in prison if convicted.

The ex-marine maintains he has been framed, saying he took a USB drive from an acquaintance thinking it contained holiday photos.

He has used earlier court hearings to appeal to journalists in English, saying that he is being mistreated in prison, not given full translations of documents and rarely given access to his lawyer.

Whelan is among high-profile detainees in Russia mooted for a potential prisoner swap with the United States.