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.
On Friday, Russia’s Foreign Minister Sergei Lavrov said Moscow is ready to discuss the topic.
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.
He could be transferred by Washington to the Russian authorities in exchange for Griner and former US Marine Paul Whelan, the reports say.
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.
Mr Kirby told reporters that the duo were being wrongfully detained and needed to be let go.
On the subject of the US proposal, Mr Kirby said: “We urge them to accept it. They should have accepted it weeks ago when we first made it.”
But according to Reuters news agency, one stumbling block is that Russia wants to add convicted murderer Vadim Krasikov, who is in prison in Germany, to the proposed swap.
When questioned about this possibility, Mr Kirby dismissed it, saying: “I don’t think we go so far as to even call it a counter-offer.”
Griner, 31, told the court she had made an “honest mistake” and had not intended to break the law.
Considered one of the best female players in the world, she was detained in February at an airport near Moscow when vape cartridges containing cannabis oil were found in her luggage. She had come to Russia to play club basketball during the US off-season.