•The outlet reports that the missing American lived in Minnesota and spent eight years in the US Air Force until 2017, when he began investing in shares and cryptocurrencies.
• "From what I understand, his only crime is that he is an American,"
The US state department has said it is aware of reports an American national living in Ukraine is being held captive by pro-Russian separatists.
Suedi Murekezi, 35, a Rwandan-born crypto investor, is said to have been arrested last month in the Russian-occupied port city of Kherson.
Friends and family say the US Air Force veteran has been falsely accused of joining pro-Ukraine protests.
He is reportedly in the same jail as two US fighters captured last month.
The state department said on Wednesday it was "aware of these unconfirmed reports".
But it declined to provide further comment "due to privacy considerations".
Mr Murekezi's family has been in daily contact with the agency, according to the Guardian.
The outlet reports that the missing American lived in Minnesota and spent eight years in the US Air Force until 2017, when he began investing in shares and cryptocurrencies.
Drawn to Ukraine by the country's liberal bitcoin regulations, Mr Murekezi made business trips to the country before permanently settling in Kherson two years ago, the Guardian said.
Sele Murekezi told the paper his brother had called on 7 July to say he was being held in the self-proclaimed, pro-Russian Donetsk People's Republic, along with two other Americans.
Andy Tai Ngoc Huynh, 27, and Alexander Drueke, 39, both of whom travelled from the US state of Alabama to join a Ukrainian volunteer unit, have been held captive since early June.
But unlike Mr Huynh and Mr Drueke allegedly, Mr Murekezi did not take part in any fighting in Ukraine, according to his family.
"They are using him as a pawn for their own propaganda purposes," his brother said, expressing concern he may be particularly poorly treated because he is black.
Project Dynamo, a US-based non-profit that evacuates American civilians from risky situations abroad, has warned Mr Murekezi could face the death penalty in Donetsk.
"From what I understand, his only crime is that he is an American," said co-founder Bryan Stern.
Last month, two British men and a Moroccan man were sentenced by a court in Donetsk to be executed by firing squad.