Paul Whelan is a former US Marine arrested on spying charges in Moscow. (File)

A former US Marine arrested on spying charges in Moscow says he is subjected to abuse and harassment in jail, but a diplomat warned Saturday a potential prisoner swap will not happen before a conviction.

Paul Whelan has been held under arrest in Russia since late December and is accused of being caught red-handed "carrying out an act of espionage".

A Moscow court on Friday extended his pre-trial arrest for another three months, with Whelan addressing the media from his cage in an emotive speech accusing Russian authorities of treating him like a "prisoner of war".

"I've been threatened, my personal safety has been threatened, there are abuses and harassment that I am constantly subjected to," Whelan said according to a BBC video.

He said he'd been deprived of showers, medical treatment, correspondence and books.

"Everything is being kept from me. This is typical prisoner of war, chapter one isolation technique," Whelan said. "They are trying to run me down so that I talk to them."

The defence team for Whelan, who also carries Canadian, Irish and British passports, has argued that he was framed by an acquaintance who passed him a USB drive that was supposed to contain vacation photos.

His lawyer Vladimir Zherebenkov said there haven't been threats on his life, just poor arrest conditions and a threat of false conviction.

Whelan is among high-profile detainees mooted for a potential prisoner swap, possibly with Maria Butina who was sentenced to 18 months' jail in the US last month after admitting to one count of conspiracy.

But a high-ranking Russian diplomat on Saturday said swapping Whelan for Butina is "out of the question" and not on the table.

"(Whelan) has not been convicted yet," Deputy Foreign Minister Sergei Ryabkov was quoted by Russian agencies as saying.

The US Embassy in Moscow slammed the court's decision to extend Whelan's arrest on Friday.

"Paul has been detained already for five months, and there is no evidence of any wrongdoing," it said in a statement.