Reed, 29, was found guilty and sentenced to nine years behind bars in July last year for allegedly hitting a Moscow law enforcement officer while drunk in 2019.

During a call with Russian Foreign Minister Sergei Lavrov earlier this month, US Secretary of State Antony Blinken demanded his release and that of another former US marine jailed in Russia.

At an appeal hearing in Moscow on Wednesday, the court ordered a psychological and psychiatric evaluation be carried out because "Reed does not remember anything about the circumstances of the event".

Russian news agencies reported that the court additionally cited a need for the exam since Reed had "consumed one litre of vodka" prior to his detention.

US Ambassador John Sullivan was present at the hearing, US Embassy spokeswoman Rebecca Ross wrote on Twitter.

"We will continue to follow this case very, very closely. My role is to protect his interests and to advocate for him," she cited the ambassador as saying.

At the time of the trial, Reed's legal representation and his family pointed to what they said were inconsistencies in the process and speculated his arrest could be politically motivated.

His case was overshadowed by the trial against another former marine, Paul Whelan, who was convicted on espionage charges in June last year and sentenced to 16 years behind bars.

Whelan's lawyer has said on several occasions that Russian authorities were negotiating a prisoner exchange for the two former US military men.

US diplomats have previously raised concerns over a case against Michael Calvey, a high-profile American investor who was arrested last year for allegedly defrauding a Russian bank.