A negotiator working on a potential agreement to return the WNBA's Brittney Griner and Marine veteran Paul Whelan to America for two Russia prisoners held by the U.S. remains "cautiously optimistic."

"I am cautiously optimistic on the Griner-Whelan negotiations," former New Mexico Gov. Bill Richardson, who also served as the U.S. ambassador to the United Nations, told CNN's "State of the Union." "I am cautiously optimistic. I think it's going to be a two-for-two. I don't want to get into who I met with. It was senior Russian officials, individuals close to President Putin."

Griner and Whelan are imprisoned in Russia, while President Joe Biden is warning of potential nuclear "armageddon" amid Vladimir Putin's staking claim to four more regions in Ukraine and suggesting the use of tactical nuclear weapons might be in play.

"I got the sense that the Russian officials that I met with that I have known over the years are ready to talk," Richardson told CNN's Jake Tapper. "And my hope right now is that, because of the nuclear situation, there, that there should be nuclear mitigation, risk talks. We should talk about the nuclear reactors, about children, about humanitarian issues, humanitarian corridors, do something about the International Atomic Energy Agency dealing with the nuclear reactors, POWs on both sides.

"I got a good sense from the Russians that the vibrations — but I'm not a government official. And I was just there primarily to deal with the prisoner issue. And we have other Americans that are there."

Griner was sentenced to nine years for drug possession, while Whelan is serving a 16-year sentence on espionage charges in 2020.

Richardson has defied U.S. warnings on American citizens staying in Moscow, but he said the final decision on a prisoner swap is ultimately solely in the hands of Biden.

"I'm not part of the government, the government channel, I have always made that clear; I respect that," Richardson said. "I think any decision, for instance, a release, a prisoner exchange, has to be made by the president.

"And I think the administration has done a good job on that. They recently did that in Venezuela. I think they're very serious about Brittney Griner and Paul Whelan.

"But I don't work for them, and I think that little misunderstanding has been cleared up."

Richardson did take a shot at "nervous Nellies" in the Biden administration not respecting his ability to negotiate the release of American citizens in Russia.

"There are a lot of nervous Nellies in the government that think they could know it all, and that's not the case: Look at my track record over 30 years," Richardson concluded.

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