Whelan had "emergency hernia surgery" after experiencing "severe abdominal pain" overnight, his brother David Whelan said in a statement, citing information from the US Embassy.

The operation was successful and Whelan was to be transferred back the same day to Moscow's Lefortovo prison where he is being held, his brother said.

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.

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.