A former US Marine imprisoned in Russia on charges of espionage expressed disappointment Thursday that more has not been done to secure his release.

“I am greatly disappointed that more has not been done to secure my release, especially as the four-year anniversary of my arrest is coming up,” Paul Whelan told CNN in a phone call shortly after US WNBA star Brittney Griner was freed in a prisoner swap with Russian arms dealer Viktor Bout.

“I was arrested for a crime that never occurred,” Whelan said.

The US earlier offered a swap deal that included Griner and Whelan, who has been held in Russia since 2018 on espionage charges, in exchange for Bout's release. But Whelan was not part of the deal that happened Thursday.

President Joe Biden said the US will continue to work for his release, maintaining: "We've not forgotten about Paul Whelan, who has been unjustly detained in Russia for years."

"This was not a choice of which American to bring home," Biden said. "Sadly, for totally illegitimate reasons, Russia is treating Paul's case differently than Brittney's. And while we have not yet succeeded in securing Paul's release, we are not giving up. We will never give up. We remain in close touch with Paul's family, the Whelan family, and my thoughts and prayers are with them today."

Asked by reporters during a press conference Thursday, White House spokesperson Karine Jean-Pierre said the Russians have treated the Paul Whelan situation “very differently” and in a “totally illegitimate way.” (Anadolu Agency)