The US embassy in Moscow said Whelan had "emergency surgery" after Russia delayed medical treatment, despite its repeated requests for him to see an English-speaking doctor from outside his prison.

"Russian authorities waited until #Paul Whelan's condition was life-threatening to provide him medical attention. That is unacceptable and dangerous," tweeted US Embassy spokeswoman Rebecca Ross.

Whelan had "emergency hernia surgery" after experiencing "severe abdominal pain" overnight, his brother David Whelan said in a statement.

The US Embassy and Russian foreign ministry confirmed Whelan had undergone an operation without giving details.

The 50-year-old was detained in Moscow in December 2018 for allegedly receiving state secrets, but he insists he was framed when he took a USB drive from an acquaintance thinking it contained holiday photos.

His trial has caused tensions between Moscow and Washington and sparked speculation over a possible prisoner swap.

The Russian foreign ministry said that Whelan had undergone a "simple operation" that he had previously refused.

The ministry said in December that Whelan had "no threatening health conditions".

The prisoner was discharged Friday and transferred back to Moscow's Lefortovo jail where he is being held, the ministry said.

US Ambassador John Sullivan spoke to Whelan as he was being transferred, the embassy said.

Whelan holds US, Canadian, Irish and British citizenship.

The former marine was head of global security at an US auto-parts supplier at the time of his arrest, when he said he was visiting Russia to attend a wedding.

His case is being heard behind closed doors at a Moscow court ostensibly due to the case involving state secrets.

The US Embassy has described the secret trial as making a "mockery of justice."

Prosecutors on Monday asked the judge to sentence Whelan to 18 years in a strict regime penal colony, while the maximum possible sentence is 20 years.

The judge will deliver a verdict June 15.

Whelan has previously complained that prison staff were refusing medical care for his hernia. His lawyer said in October that he was offered an operation but decided not to go through with it.

His brother David on Friday voiced concern that "the coronavirus is spreading" through the prison where Whelan is being held and he could be "more susceptible" to illness as he recovers.

The Russian foreign ministry said he is "receiving all the necessary specialised medical assistance where he is being held".