Russia said on Thursday there was no deal yet with the United States on swapping detained U.S. basketball star Brittney Griner and a former Marine for an imprisoned Russian dubbed by American prosecutors one of the world's most prolific arms dealers.

Secretary of State Antony Blinken said on Wednesday the United States has made a "substantial offer" to Russia to release American citizens held in Russia. A source said that Washington was willing to exchange convicted arms trafficker Viktor Bout, known as the "Merchant of Death."

Kremlin spokesman Dmitry Peskov expressed surprise at the public remarks from Blinken. Russia has cautioned the United States that such discussions are best conducted in private.