The United States on Wednesday formally protested over the arrest in Russia of former US marine Paul Whelan, who has been jailed on charges of espionage since December.
The US embassy in Moscow has asked that allegations of mistreatment in pre-trial detention be investigated and Whelan's safety guaranteed, according to spokeswoman Andrea Kalan.
"We have protested to the (foreign ministry) Paul Whelan's mistreatment while in custody," Kalan wrote on Twitter.
"Welfare of US citizens abroad is our highest priority," she added.
The Russian ministry responded via its own Twitter account, suggesting the US instead comment on the rights of incarcerated Russian citizens on its territory.
The US note of protest comes a few days after Whelan's latest appeal of arrest conditions was rejected.
Whelan, who was arrested in December after allegedly receiving state secrets, maintains that he has been framed and that he took a USB drive from an acquaintance thinking it contained vacation photos.
During his most recent 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. He also holds Canadian, Irish and British passports.
His relatives in the US have mounted a campaign for his release. His sister met National Security Advisor John Bolton and top Russia advisor Fiona Hill earlier this month, according to a statement by his brother David Whelan.
Whelan has complained of pressure and said Russian authorities are isolating him to coerce a confession due to lack of evidence of his involvement in espionage.