Ukrainian President Volodymyr Zelenskyy arrived for his first NATO summit meeting in Lithuania's capital on Wednesday where he received a jovial welcome from world leaders eager to shake his hand -- a mood in keeping with the more conciliatory tone Zelenskyy has struck since landing in Vilnius.

The Ukrainian leader ultimately made a beeline for U.S. President Joe Biden, as NATO Secretary General Jens Stoltenberg created a path for him through the crowd of world leaders. The two men -- Biden in a suit and Zelenskyy in a dark green polo -- greeted each other with smiles and shook hands before chatting briefly.

Everyone eventually took their seats for the meeting of the NATO-Ukraine Council -- the first of its kind -- and Stoltenberg began his remarks with a warm welcome to Zelenskyy, whom he called "our dear friend."

"Welcome to this first meeting of the NATO-Ukraine Council," Stoltenberg said. "This is truly a historic moment, allies and Ukraine sitting side-by-side as equals to address our common vision of Euro-Atlantic security."

"And I'm really honored to welcome in our midst our dear friend, Volodymyr Zelenskyy, president of Ukraine. Welcome to Lithuania and it's great to have you here," he added.

Stoltenberg proceeded to give a very clear message about where NATO stands on Ukrainian membership, though no details on the pathway as has been a point of contention. He noted the summit has "reaffirmed that Ukraine will become a member of the alliance and we have made decisions to bring Ukraine closer to NATO."

"Today, we meet as equals," he said. "And I look forward to the day we meet as allies."

-ABC News' Cheyenne Haslett

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.

Ukrainian President Volodymyr Zelenskyy and U.S. President Joe Biden are set to meet in Lithuania’s capital on Wednesday afternoon, a day after NATO leaders announced during a summit that Ukraine will be allowed to join the alliance "when allies agree and conditions are met" but didn't offer a timeline.

Earlier Wednesday, Zelenskyy held a joint press conference with NATO Secretary-General Jens Stoltenberg, fielding many questions from reporters about Ukraine's path to NATO membership. The Ukrainian president took a noticeably softer tone compared to his remarks the previous day criticizing the lack of a timeline as "unprecedented and absurd."

Zelenskyy told reporters it's difficult as NATO partners are living under different conditions, whereas in Ukraine "survival" matters. He said he understands some people are "afraid" to talk about Ukraine joining NATO because "nobody is willing to have a world war." He acknowledged that his country cannot be a member of the alliance while a war is going on within its borders, but he said "signals are important."

When asked about his upcoming meeting with Biden and how he plans to convince the U.S. president that Ukraine is ready for NATO membership, Zelenskyy responded with gratitude to the United States and confidence that Ukraine will join the alliance once Russia's war is over.

"I'm grateful to President Biden and to the Congress and to the people of United States that are truly the leaders in support and assistance to Ukraine. We highly appreciate this," the Ukrainian president told reporters. "Not planning to find any arguments for making sure that President Biden would see us in NATO. I believe that those arguments, they should be mutual because it's all about this security, the East, the European continent, the Eastern Flank of NATO. And I believe that NATO needs us just as we need NATO. And I believe that this is absolutely fair. I am confident that after the war, Ukraine will be in NATO. We will be doing everything possible to make it happen so that we with the United States would have a same understanding and same vision."

-ABC News' Molly Nagle and Joe Simonetti

The United States is standing by the NATO communique released Tuesday and its language around Ukraine joining the alliance, despite criticism from Ukrainian President Volodymyr Zelenskyy over the lack of a timeline to do so.

"The United States clearly joined with NATO allies in agreeing to a strong positive message reaffirming that Ukraine will become a member of the alliance," U.S. National Security Council Senior Director for Europe Amanda Sloat said during a press briefing in Lithuania's capital on Wednesday morning, on the final day of a high-stakes NATO summit. "And as the communique has made clear, as the president has spoken to directly in the past, we recognize that Ukraine has already made significant progress in terms of reforms. That was part of what led to allies making the decision to say that the Membership Action Plan was no longer required for Ukraine."

"But as both the president has said and as the communique made clear, there is still the need for Ukraine to take further democratic and security sector reforms," she added.

Sloat told reporters that the U.S. has been and would continue to work with Ukraine both bilaterally and through the NATO alliance to ensure that the reforms required to join the alliance are met.

When asked to respond to Zelenskyy's criticism that the lack of a timeline was "unprecedented and absurd," Sloat defended the agreement as a significant one.

"I would agree that the communique is unprecedented, but I see that in a positive way. We joined with allies yesterday in agreeing to a very strong, positive message. We reaffirmed that Ukraine will become a member of the NATO alliance," she said, arguing that removing the Membership Action Plan requirement for Ukraine was a "very significant" step on NATO's part.

-ABC News' Molly Nagle

Ukrainian Armed Forces have yet to reach their "full potential," but two top generals exclusively told ABC News the counteroffensive is "going to plan" despite concerns from Western analysts that Ukraine is not making enough progress.

Gen. Oleksandr Tarnavskyi, who is leading the counteroffensive in the south, told ABC News' chief global affairs correspondent Martha Raddatz that the situation is "stable."

"The main thing is that we haven’t reached our full potential," Tarnavskiy said.

Ukrainian troops launched the counteroffensive a month ago, attacking on multiple axes on the southern frontline in Zaporizhzia using Western-supplied vehicles. Ukraine succeeded in piercing Russian lines at two points, liberating a string of villages, but has since been locked in ferocious fighting

Gen. Oleksandr Syrskyi, commander of the Ukrainian ground forces, told Raddatz that last month's failed rebellion by the Putin-backed PMC, the Wagner Group, wouldn't impact the fighting on the ground.

"It would be better for us if there were some negative consequences in Russia itself, but it doesn't matter for me," he said.

Asked if he was confident of retaking the key city of Bakhmut, Syrskyi said, "Yes, of course. I’m sure."

Both sides have suffered heavy casualties, according to Western officials, particularly in the south where Russia has so far conducted "relatively effective defensive operations" in the southeastern Zaporizhzhia province, the U.K. Ministry of Defense reported.

"The enemy is suffering eight times or even 10 times higher losses, especially with the number of killed troops," Syrksyi said.

-ABC News' Guy Davies and Meghan Mistry