An in-person meeting between Ukrainian President Volodymyr Zelenskyy and U.S. President Joe Biden is currently underway in Lithuania's capital.

Zelenskyy took to Twitter on Wednesday evening to announce that their bilateral meeting in Vilnius "has begun." It's the third face-to-face meeting to take place between the two leaders this year.

"The focus is on security and everything that enhances it," Zelenskyy tweeted. "It will be meaningful."

In a joint declaration released Wednesday, the leaders of the Group of Seven reaffirmed their "unwavering commitment to the strategic objective of a free, independent, democratic and sovereign Ukraine."

"We will stand with Ukraine as it defends itself against Russian aggression, for as long as it takes," the joint declaration states.

G-7 leaders wrote in the joint declaration that they are working to "formalize" their "enduring support to Ukraine as it defends its sovereignty and territorial integrity, rebuilds its economy, protects its citizens, and pursues integration into the Euro-Atlantic community."

They also said they will "work with Ukraine" on an "enhanced package of security commitments and arrangements in case of future aggression to enable Ukraine to defend its territory and sovereignty."

The G-7 is an intergovernmental political forum consisting of Canada, France, Germany, Italy, Japan, the United Kingdom and the United States.

-ABC News' Ellie Kaufman

Ukrainian President Volodymyr Zelenskyy applauded the "outcome" of the annual NATO summit on Wednesday, describing the new commitments to his country as a "significant security victory."

"Ukrainian delegation is bringing home significant security victory," Zelenskyy said in brief remarks alongside world leaders in Lithuana's capital. "For our country, for our people, for our children."

Zelenskyy's laudatory tone was a sharp departure from his comments on Twitter the previous day. He told reporters that the long-term commitments announced by G-7 leaders on Wednesday would "absolutely" open "new security opportunities" and that he was "grateful."

"I thank everyone who made it possible," he added.

-ABC News' Cheyenne Haslett

G-7 leaders including U.S. President Joe Biden stood alongside Ukrainian President Volodymyr Zelenskyy in Lithuania's capital on Wednesday to announce a new long-term security package for Ukraine's future, while reaffirming that Kyiv has a place in NATO -- but not yet.

"All our allies agreed Ukraine's future lies in NATO. That's not a surprise to any of us, I don't think. I hope it's not a surprise to you, Mr. President," Biden said, turning to Zelenskyy during the press conference in Vilnius.

"Ukraine continues to make progress on necessary reforms," he added. "But we're not waiting for that process to be finished to make the long-term commitments that we're making to Ukraine security."

The U.S. commander in chief said he and Zelenskyy discussed what guarantees could be made to Ukraine during his visit to Kyiv earlier this year.

"Today, the members of the G-7 are launching the Joint Declaration of Support for Ukraine to make it clear that our support will last long into the future," Biden told reporters. "This starts the process by which each of our nations and any other nation who wishes to participate will negotiate long-term bilateral security commitments with and to Ukraine."

"We're going to help Ukraine build a strong, capable defense across land, air and sea," he added, "from which will be a force of stability in the region and deter against any and all threats."

-ABC News' Molly Nagle

In the wake of Saturday's short-lived attempted rebellion against the Kremlin by the Wagner private military company, Mikhail Khodorkovsky, an exiled oligarch considered one of Putin’s best-known opponents, told ABC News he supports the mutiny and encourages Russians to back the leader of the mercenary group.

Once Russia’s richest man, Khodorkovsky, a Putin opposition activist, spent 10 years imprisoned after he challenged Putin, his case now considered a foundational moment for Putin’s regime.

When Wagner founder Yevgeny Prigozhin and his fighters marched on Moscow Saturday before making a sudden about-face, Khodorkovsky was notable among Russia’s pro-democracy opposition in calling on people to support Prigozhin, arguing that allowing him to remove Putin would create an opportunity for the democrats.

Khodorkovsky told ABC News he believed Prigozhin’s actions were a real coup attempt and that it had “seriously undermined” Putin’s power. He predicted that similar opportunities to collapse the regime will be launched soon.

"The blow to Putin’s reputation, to the authorities’ reputation, was absolutely fantastical," Khodorkovsky said. "Putin’s government today is, without a doubt, strongly undermined by what happened -- his authority, his ability to control the security services is seriously undermined."

Khodorkovsky said Prigozhin’s march on Moscow had undermined Putin’s popularity, showing neither ordinary Russians nor the security services were prepared to act to protect him.

“Along the entire route of Wagner's columns, no one in any way tried to hinder him (Prigozhin). Even the security forces did not try to stop him," Khodorkovsky said. “It showed that, in fact, inside the country, Putin has an absolute void."

Khodorkovsky said he did not support Prigozhin himself -- considering him a "war criminal" -- but that the democratic opposition should have sought to help him overthrow Putin, and then taken power from him after.

Khodorkovsky criticized other parts of the anti-Kremlin opposition who attacked him for calling on people to assist Prigozhin, saying he believed the opposition had “slept through” the opportunity and suggesting it should have sought to stage a rebellion in Moscow at the same time.

"There will definitely now be more such opportunities because of Putin’s weakening. But the next time we need to simply be more ready," said Khodorkovsky, who is living in exile in England. "If an uprising had started in Moscow to meet Prigozhin then a situation could have developed quite differently.

ABC News' Patrick Reevell