Bill Richardson, the former New Mexico governor and ambassador to the United Nations, said Sunday that he is "cautiously optimistic" that a deal will be reached to release Brittney Griner and Paul Whelan, two Americans imprisoned in Russia.

Richardson ā€” who has been unofficially negotiating with Russian officials as a private citizen ā€” told CNN's "State of the Union" that a deal could be feasible by the end of the year, saying that Russia appeared to be willing to move forward on an agreement to exchange Griner, a WNBA all-star, and Whelan, a former Marine, for two Russian prisoners held by the United States.

"I got the sense that the Russian officials that I met with, that I've known over the years, are ready to talk," Richardson said.

Griner, who was arrested just before Russia invaded Ukraine in February, was sentenced to nine years in a Russian penal colony after pleading guilty to a drug possession charge. Whelan was sentenced to 16 years on espionage charges in 2020.

The White House has recently ramped up efforts to secure their release. Last month, President Joe Biden and his national security adviser, Jake Sullivan, met with members of Whelan's family and Griner's wife, Cherelle Griner, and her agent, Lindsay Kagawa Colas.

But without much public progress in the negotiations, members of Griner's family have also looked for help from Richardson, who has previously worked behind the scenes to secure the release of American prisoners from countries such as Iran, North Korea and Sudan.

Richardson's unofficial diplomacy has put him at odds with the Biden administration as relations between Washington and Moscow have deteriorated over Russia's aggression in Ukraine.

He has also drawn criticism from human rights advocates, who have said his trips ā€” such as his meeting with members of Myanmar's military junta last year ā€” lend legitimacy to authoritarian governments and distract from the White House's official efforts.

On Sunday, CNN's Jake Tapper asked Richardson to respond to comments last month from John Kirby, the former Pentagon press secretary, in which he implored Richardson and other private citizens not to negotiate on behalf of the U.S. government.

"I respect that. I think any decision, for instance, release of prisoner exchange, has to be made by the president," Richardson said.

But he insisted that there was still value for him and other like-minded individuals to continue pushing for Griner's release in private channels.

"There are a lot of nervous Nellies in the government that think they can know it all, and that's not the case," he said. "Look at my track record over 30 years ā€” I'm going to continue these efforts."

This article originally appeared in The New York Times.