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U.S. Secretary of State Antony Blinken revealed for the first time Thursday that the United States has put forward a proposal to secure the release of Paul Whelan, the former U.S. Marine in Russian custody for more than four years. 

Blinken revealed the news during a press conference in New Delhi, India where he was attending the G-20 conference with other foreign ministers. 

Antony Blinken G-20 India

U.S. Secretary of State Antony Blinken speaks during a press conference on the sidelines of the G20 foreign ministers meeting, Thursday, March 2, 2023, in New Delhi. (AP Photo/Altaf Qadri)

Blinken said he ran into his Russian counterpart Sergei Lavrov for the first time since Russian President Vladimir Putin launched his invasion of Ukraine more than a year ago.

"The United States has put forward a serious proposal. Moscow should accept it," Blinken said, without elaborating. "We’re determined to bring Paul and every other American citizen who’s unjustly detained around the world home. We won’t rest until we do." 

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State Department spokesman Ned Price said earlier Thursday that Blinken’s proposal was not the first time Russia has heard it. 

"What [Lavrov] did hear today was once again a strong statement from the Secretary of State that Russia should accept this proposal and, in turn, it should release Paul Whelan," Price said, without going into details of the proposal so as not to jeopardize U.S. efforts. 

either Griner or Whelan

NBC News issued an embarrassing correction after reporting President Biden could have received Paul Whelan, instead of WNBA star Brittney Griner, in the White House’s controversial trade for the "Merchant of Death." (Sofia Sandurskaya, Moscow News Agency photo via AP/AP Photo/Alexander Zemlianichenko)

Whelan, a former U.S. Marine, has been in Russian custody since Dec. 28, 2018, convicted on charges of espionage and spying for the U.S. government and sentenced to 16 years in prison. Whelan and the U.S. have denied charges as the 52-year-old remains imprisoned at a labor camp in Russia’s Mordovia Republic. 

Whelan’s case has been compared to that of WNBA star Brittney Griner, who was released from Russian custody last December after spending less than a year in prison. Griner was arrested in February after authorities discovered vape cartridges containing a small amount of cannabis oil in her luggage. 

Lavrov G20

Russian Foreign Minister Sergey Lavrov smokes as he walks to attend the second session of the G20 foreign ministers meeting, in New Delhi, India, Thursday, March 2, 2023.  (AP Photo/Manish Swarup)

The Biden administration had initially sought the release of both Griner and Whelan in exchange for notorious arms dealer Viktor Bout, a Russian citizen serving a prison sentence in America. Known colloquially as the "Merchant of Death," he was convicted in 2011 of conspiracy to kill Americans and sentenced to 15 years in prison. 

The Biden administration ultimately agreed to exchange Bout for Griner alone, leaving Whelan behind in Russia. 

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"Whether the Biden Administration can get Paul Whelan out of Russia or not is dependent on whether the  US and Russia's definitions of a ‘serious proposal’ align or not. The Russians want to get Col. Vadim Krasikov out who is serving out a life sentence for murder in Germany," former Intelligence Officer Rebekah Koffler told Fox News Digital. 

"Since it’s not within the US jurisdiction, it would be complicated for the U.S. government to pull it off. Giving up Krasikov, a Russian national, former FSB colonel who murdered a Georgian citizen in broad daylight in 2019 in Germany would signal to the Russians that they can just assassinate targets at will outside of Russia and then get the assassins back in a prisoner swap operation."