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

The United States "is prepared to defend every inch of NATO territory," National Security Advisor Jake Sullivan said Wednesday, amid Russia's threats of military retaliation if Sweden and Ukraine officially join the alliance.

"At the end of the day, Russia does not get a vote in who joins NATO," Sullivan told ABC News' George Stephanopoulos during an interview Wednesday on "Good Morning America."

"If Russia does choose to test Article 5, President Biden has said he is prepared to defend every inch of NATO territory," he added. "And later tonight, when he addresses a large crowd in Vilnius in a speech about his vision for Euro-Atlantic security and for American leadership, he will reinforce this point about the sanctity of Article 5."

Article 5 of the NATO treaty states that the signed parties "agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all."

Sweden and Finland, which have historically embraced neutrality, applied for NATO membership together last year despite warnings from Moscow against doing so, as Russian forces invaded Ukraine. Finland officially joined the alliance in April following Turkish ratification, but Turkey and Hungary have yet to ratify Sweden’s membership. NATO Secretary-General Jens Stoltenberg announced Monday that Turkish President Recep Tayyip Erdogan has agreed to send Sweden’s NATO accession documents to Turkey’s Parliament for approval "as soon as possible," one of the last steps in the membership process.

Meanwhile, Ukraine is pushing hard for NATO membership as Russia’s war presses on. Stoltenberg announced Tuesday that the bloc "will issue an invitation for Ukraine to join NATO when allies agree and conditions are met," pledging to speed up the process by eliminating bureaucratic hurdles, but without offering a timeline.

"All 31 allies got together yesterday to say with one voice that Ukraine's future is in NATO. The real question is just how we work down the pathway to get them in," Sullivan told ABC News on Wednesday. "So the signal that NATO sent yesterday was very positive to Ukraine about its future prospects for joining NATO, and we will work in terms of the democratic and security sector reforms that are necessary. And in the meantime, we're not just going to sit around. We're going to provide Ukraine with the weapons and military assistance it needs to defend its territory against Russia and to deter future aggression from Russia."

Ukraine has requested F-16 fighter jets and long-range missiles to aid its counteroffensive against Russian forces. Sullivan said the U.S. has already taken steps with NATO allies to begin the training of Ukrainian pilots on F-16s, which "will take some time," and then the aircraft will be transferred to Ukraine "likely from European countries that have excess F-16 supplies." As for the long-range missiles, Sullivan said the U.S. will continue to look into the issue and discuss it with Ukraine.