The US has urged Moscow to accept a deal to free basketball player Brittney Griner, who has been sentenced to nine years in a Russian prison.
The double Olympic winner was convicted of possessing and smuggling drugs after admitting to possessing cannabis oil.
White House national security spokesman John Kirby said the US offer was “a serious proposal”, but gave no details.
But US media reports suggest Washington is offering a prisoner swap involving a Russian arms trafficker.
Viktor Bout known as the “merchant of death” is serving a 25 year-prison sentence in the US.
Report says he could be transferred by Washington to the Russian authorities in exchange for Griner and former US Marine Paul Whelan.
Whelan, who has US, British, Canadian and Irish passports, was sentenced in 2020 to 16 years in jail in Russia after being convicted of spying.
Kirby told reporters that the duo were being wrongfully detained and needed to be let go.