U.S. Secretary of State Antony Blinken and his Russian counterpart Sergey Lavrov spoke for the first time since the Russian invasion of Ukraine to accept a U.S. proposal for the release of two Americans detained in Russia.

Blinken declined to outline Foreign Minister Lavrov’s response to the "significant proposal" the U.S. put forward weeks ago, but said it was "important" that Lavrov heard directly from him on the matter.

"I urged Foreign Minister Lavrov to move forward with that proposal," Blinken said, describing the conversation as "frank and direct."

At issue is the release of Brittney Griner and Paul Whelan, two Americans who remain in Russian custody.

The Biden administration has faced political pressure to free Griner and other Americans whom the United States has declared to be "wrongfully detained" – a designation sharply rejected by Russian officials.

Whelan, a corporate security executive from Michigan, was sentenced to 16 years in prison on espionage charges in 2020. He and his family have vigorously asserted his innocence. The U.S. government has denounced the charges as false.

Russia has for years expressed interest in the release of Viktor Bout, a Russian arms dealer once labeled the "Merchant of Death." He was sentenced to 25 years in prison in 2012 on charges that he schemed to illegally sell millions of dollars in weapons.

Griner, 31, is a basketball player who was arrested in February after Russian authorities said they found marijuana oil in her possession. She pleaded guilty earlier in July to drug charges but has maintained she used medicinal cannabis to treat pain and denied she intended to violate Russian law.

The U.S. has not offered details about its proposal but CNN reported on Wednesday that the Biden administration offered to swap convicted Russian arms trafficker Bout for the two Americans.

Bout is currently serving a 25-year prison sentence and the Justice Department has voiced opposition to the potential exchange, according to CNN.

In addition to the U.S. offer, Blinken said he "made clear" to Russia that the possible annexation of additional Ukrainian territory "would never be accepted."

"The world will not recognize annexations. We will impose additional significant costs on Russia if it moves forward with its plans," he said.

"We’ll also continue to stand with Ukraine, support its ability to defend itself, and impose costs on Russia until it ends its aggression,"​​​​​​​ he added.