"On December 28, 2018, Russian authorities arrested Paul Whelan," US embassy spokesperson Rebecca Ross wrote on Twitter. "365 days. Still no evidence. Still no phone calls to his family despite multiple requests. Still no independent medical care. Enough is enough. Let Paul go home THIS year!"
Whelan -- a former US Marine who is also a citizen of the UK, Canada and Ireland -- was detained at a Moscow hotel last year by Russian authorities who alleged he was involved in an intelligence operation. He has been in Lefortovo prison in Moscow since.
Whelan denies the spying charges and says he has been denied proper medical treatment. His family maintains he traveled to Moscow to attend a wedding and was arrested on false charges.
A Russian court this week extended Whelan's detention until March 29 at the request of investigators from Russia's Federal Security Service.
Whelan's brother David told CNN in July that his sibling, who has appeared visibly thinner in court appearances, needed better medical oversight and was only getting "first aid level" assistance.
Earlier this year, the Russia Foreign Ministry raised the possibility that Konstantin Yaroshenko, convicted of drug smuggling in 2011 and sentenced to 20 years in prison, could be returned "in exchange for any American national" held in Russia. Yaroshenko is serving his sentence at the Federal Correctional Institution in Danbury, Connecticut.
Another name speculated for a swap was Maria Butina, a Russian gun rights activist who was imprisoned in the US for acting as a foreign agent until her release in October. Viktor Bout, a Russian arms dealer dubbed the "Merchant of Death" who was sentenced to 25 years in US Federal Prison in 2012, also has been mentioned.
David Whelan said in July that the Russian remark about a prisoner exchange "is a clear statement that Paul is a political pawn and that they're hoping to extract some sort of value from the United States by trading him for something."