Russia-Ukraine updates: Putin suspends key US-Russia nuclear treaty in speech denouncing West

U.S. Secretary of State Antony Blinken on Thursday told the United Nations Security Council that referendums in Russia-backed regions of Ukraine are part of the Kremlin's "diabolical" plan.

Blinken alleged that Russia plans to bus in Russians to replace Ukrainians in the eastern and southern regions still under its control and call for a vote. He warned that Russia will "manipulate the result to show near unanimous support for joining the Russian Federation."

"This is right out of the Crimea playbook," Blinken said of Ukrainian territory Russia annexed in 2014. "As with Crimea, it's imperative that every member of this council, and for that matter every member of the United Nations, reject the sham referenda and unequivocally declare that all Ukrainian territory is and will remain part of Ukraine."

He said no Russian claim to annexed territory "can take away Ukraine's right to defend its own land."

Images are emerging showing Wednesday's prisoner exchange between Ukraine and Russia.

According to Ukrainian officials, the photos and videos surfacing Thursday show the prisoners of war exchange that occurred in Chernihiv, in northern Ukraine.

The prisoner exchange included two Americans who were being held captive by Russian-backed forces after volunteering to fight with Ukrainian forces, their families said.

Alexander Drueke and Andy Huynh, both military veterans from Alabama, were reported missing by their families following a fight in the Kharkiv area of Ukraine in June.

Drueke and Huynh were among 10 foreign prisoners of war released following a mediation by Saudi Crown Prince Mohammed bin Salman, the Saudi foreign ministry said.

"We are thrilled to announce that Alex and Andy are free. They are safely in the custody of the US embassy in Saudi Arabia and after medical checks and debriefing they will return to the States," the families of Drueke and Huynh said in their joint statement.

British prisoners of war also posed for a photo on a plane following their release from Russian captivity, according to Reuters.

Other images released by State Security Service of Ukraine showed Ukrainian soldiers smiling after they were released in the Chernihiv region.

Meanwhile, the Russian defense ministry press service released an image from a video of Russian war prisoners walking off a plane in an unspecified location in Russia. Russia said 55 of its troops were released in a prisoner exchange.

Ukrainian officials said 215 of its soldiers and foreign citizens were freed from captivity in Russia.

During Thursday's United Nations Security Council meeting in New York City, U.S. Secretary of State Antony Blinken is expected to urge all members to send a clear message of opposition to Moscow over Russian President Vladimir Putin's recent threats of nuclear warfare, according to a senior official with the U.S. Department of State.

The State Department official previewed what Blinken will say at the upcoming session, which his Russian counterpart Sergey Lavrov is expected to attend. While Blinken plans to tell the council that the United States takes Putin's nuclear threats seriously, he is not expected to urge any specific action, given the obstacles that the council's makeup presents. Rather, the official said Blinken sees Thursday's meeting as an opportunity to further shine a spotlight on the impacts of Russia's ongoing war in Ukraine.

Furthermore, Blinken is expected to hit on the latest developments out of Russia, including the partial military mobilization and referenda. He also plans to reference evidence of atrocities uncovered in recent days, specifically in the eastern Ukrainian city of Izyum, stressing that these are not the actions of rogue units but a clear pattern emerging across Russian-occupied territory and must be met with accountability.

While Lavrov is expected to attend Thursday's meeting, there is of course no guarantee he will be in the room when Blinken speaks. Blinken, however, is expected to remain through the entirety of the session, where both Russia and China will also have an opportunity to address the room.

-ABC News' Shannon Crawford

Ukrainian President Volodymyr Zelenskyy demanded punishment for Russia in his remarks to the United Nations General Assembly on Wednesday.

"A crime has been committed against Ukraine, and we demand just punishment," he said in video remarks, the only state leader allowed to appear virtually this year.

Zelenskyy spelled out the alleged atrocities discovered in Izyum after Russian forces retreated. "The bodies of women and men, children and adults, civilians and soldiers were found there -- 445 graves," he said.

Zelenskyy vowed to other world leaders that Ukraine's forces would ultimately emerge successful -- and claimed any rhetoric from Russia about negotiating peace was a façade.

"We can return the Ukrainian flag to our entire territory. We can do it with the force of arms, but we need time," he said. "Russia wants to spend the winter on the occupied territory of Ukraine and prepare forces to attempt a new offensive -- new Buchas, new Izyums."

He warned that Russia's warfare near nuclear plants meant no one was safe and again made an appeal for Russia to be branded as a state sponsor of terrorism by all nations -- something the Biden administration has so far said it is against.

"We must finally recognize Russia as a state sponsor of terrorists, at all levels, in all countries," Zelenskyy urged. "This is the foundation for restoring global security."

-ABC News' Shannon Crawford