Former US Marine Robert Gilman, who is serving a long prison sentence in Russia, has been admitted to a hospital, Kommersant newspaper reported on Thursday.
The business daily quoted Gilman's lawyer as saying he was receiving treatment but that it was too early to talk about a diagnosis.
A US State Department spokesperson said: "We are aware that an American, Robert Gilman, is detained in Voronezh, Russia, and the Department is providing appropriate consular assistance. We take our commitment to assist Americans abroad seriously and will continue to track closely Mr. Gilman's case."
Gilman was first jailed in 2022 for assaulting a police officer while drunk, and his sentence was extended in 2024 following subsequent convictions for assaulting prison officials and a state investigator.
State media reported last December that he was facing a total sentence of 10 years after a further conviction for assaulting prison staff.
Russian media have previously cited Gilman's lawyers as saying that he originally came to Russia to study and obtain citizenship.
Gilman's supporters in the US say he was ill when he was first arrested, and that he was provoked while in prison into actions that led to the further charges.
Hearing postponed due to Gilman's illness
Kommersant said that because of Gilman's unspecified illness, a court in the southern city of Voronezh had postponed a hearing into a prosecutor's appeal against the latest sentence.
A source close to the Kremlin told Reuters last year that Gilman was one of nine people on a list of Americans jailed in Russia that Washington wanted to be released and returned to the US
The State Department spokesperson said: "The Trump Administration has no higher priority than the safety and security of Americans."