"I was injured in the prison... the prison doesn't want to tell you," Paul Whelan told journalists from a cage in a Moscow court, which is to decide on whether to extend his provisional detention.

Whelan arrived in the court handcuffed and escorted by two security guards wearing black masks and plain clothes.

"You can see the treatment," he said as he entered.

"I am standing here in great pain due to an injury sustained in the prison by the prison guards," he said, looking pale.

He did not say what type of injury he suffered.

Whelan, who also holds Canadian, Irish and British passports, was arrested in December after allegedly receiving state secrets. He risks up to 20 years in prison if convicted.

He maintains he has been framed and that he took a USB drive from an acquaintance thinking it contained holiday photos.

His family said Whelan had been in Moscow for a friend's wedding.

During an earlier hearing, he made a direct appeal to US President Donald Trump.

"Mr President -- we cannot keep America great unless we aggressively protect and defend citizens wherever they are in the world," he said.

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