The Lefortovo district court in Moscow ruled on 23 August to extend Whelan's arrest for a further two months.

Russian investigators have pressed espionage charges against Paul Whelan, his lawyer Vladimir Zherebenkov said.

"Next week familiarisation with the case files will begin", Zherebenkov said, stressing that his client denies any wrongdoing.

According to the lawyer, the pace of familiarisation will depend on a translator.

The US embassy called on Russian authorities to release Whelan so that he could travel back to the United States to receive medical attention.

The statement comes after the last hearing on 23 August, when Whelan complained about his health — he claimed that he had not eaten since the previous evening and had taken painkillers. The judge called an ambulance, but paramedics determined that he did not need hospitalisation.

Whelan, who is also a citizen of Canada, Ireland and the United Kingdom, was arrested in Russia in December on charges of espionage. If convicted, he may face from 10 to 20 years in prison.

A former US marine, Whelan has denied the charges and insisted he was in Russia to attend a friend’s wedding. Court papers have revealed that he has travelled to Russia on several occasions since 2007.