Secretary of State Antony Blinken said on Sunday that America is “actively engaged” with Russia over a possible prisoner swap to free weed-smuggler WNBA player Brittney Griner and former U.S. Marine Paul Whelan, who are in Russian custody.

“The proof will be in the pudding,” Blinken said in an interview with “Face the Nation,” when asked whether the Russians are serious about negotiations to release Griner and Whelan. “We have to see if the engagements that we’ve had, the discussions that we have, produced an actual result. That’s the most important thing, but we are not resting on the laurels of having put forward a proposal some months ago. We’ve been actively engaged over these many months to try to move things forward.”