A Ukrainian counteroffensive may conclude by the end of the summer if Ukraine receives the weapons it needs, Major General Dmytro Marchenko of the Ukrainian Armed Forces told Radio Liberty on Wednesday.

“I am just a realist… but the fact is that it will not end quickly,” Marchenko said in response to a question about when ongoing hostilities might end. “In some places we will lose time, let [the Russians] dig in more, so [the fighting] will be longer,” he added.

The Major General also stressed that a "point of no return" had been reached on the issue of peace agreements with Russia. “Why this pile of deaths?... At the very beginning, it was possible to stop all this at the stage of negotiations. But after what [the Russians] did, we passed the point of no return,” Marchenko said.

He added that Ukraine has the pace and desire to finish the war. “Just give us some weapons and we will do it.”

With the U.S. announcing $1 billion more in military aid on Wednesday, Ukrainian President Volodymyr Zelenskyy was quick to thank President Joe Biden during a phone conversation on the same day. Zelenskyy expressed gratitude to Biden for his leadership position in supporting Ukraine's struggle for freedom and for his unwavering backing of Ukraine's independence and territorial integrity.

"The security support from the United States is unprecedented. It brings us closer to a common victory over the Russian aggressor," Zelenskyy said, noting the importance and timeliness of the new military aid package.

"We also appreciate the significant financial and macroeconomic support for Ukraine from the United States. It helps us endure this war," the Ukrainian president said in reference to the $225 million in humanitarian assistance Biden pledged to Ukraine on Wednesday.

"We must win this war and return our occupied territories. This will be our common victory," Zelenskyy concluded.

NATO Secretary General Jens Stoltenberg also thanked the U.S. for its leadership in mobilizing and coordinating international security support for Ukraine that addresses the country's urgent needs for military equipment.

Speaking at the meeting of the U.S.-led Ukraine Defence Contact Group at the NATO Headquarters in Brussels on Wednesday, Stoltenberg welcomed the new round of military aid to Ukraine announced by NATO Allies. On top of the U.S. package, Germany has promised to provide Ukraine with 3 MLRSs while Slovakia pledged to deliver several helicopters and rocket launcher ammunition. Canada, Poland and the Netherlands all promised to boost the short-handed Ukrainian artillery.

“NATO as an organization is stepping up its support,” Stoltenberg said. He added that NATO Allies are committed to continue providing Ukraine with the military equipment it needs to prevail, including heavy weapons and long-range systems.

The Secretary General said that the Allies will agree on a new comprehensive assistance package for Ukraine at the Madrid Summit in late June. This will help Ukraine in the longer-term to transition from Soviet-era equipment to modern NATO weaponry and to enhance interoperability with NATO, Stoltenberg pointed out.

President Zelenskyy confirmed his attendance at the Madrid Summit in a tweet from Wednesday.

-ABC News' Edward Szekeres, Max Uzol, Yuriy Zaliznyak and Yulia Drozd

Two U.S. lawmakers said Wednesday they have been asked by the families of two former U.S. service members who volunteered to assist the Ukrainian forces for their help in locating them.

Alabama Rep. Terri Sewell said in a statement her office is helping a family locate Alexander Drueke, of Tuscaloosa, Alabama.

"Earlier this week, the mother of Alexander Drueke, a Tuscaloosa Army Veteran who volunteered to assist the Ukrainian Army in combating Russia, reached out to my office after losing contact with her son. According to his family, they have not heard from Drueke in several days," she said in a statement.

She said her office has been in contact with the State Department, the FBI and other members of the Alabama Congressional Delegation.

Alabama Rep. Robert Aderholt said his office is helping in the search for Andy Tai Ngoc Huynh, 27, of Trinity, Alabama, after his family reached out to the congressman's office this week.

"According to Huynh’s family, they have not been in contact with him since June 8, 2022, when he was in the Kharkiv area of Ukraine," he said in a statement.

Aderholt said his office has reached out to the State Department and FBI to "get any information possible."

Huynh, a former Marine, spoke to Huntsville, Alabama, ABC affiliate WAAY in April about his decision to help defend Ukraine.

"I've made peace with the decision. I know there's a potential of me dying. I'm willing to give my life for what I believe is right," he told the station.

White House spokesman John Kirby told reporters Wednesday afternoon that he "can't confirm the reports" of two Americans captured in Ukraine.

"We'll do the best we can to monitor this and see what we can learn about it," he said. "Obviously, if it's true, we'll do everything we can to get them safely back home."

The State Department also is aware of the "unconfirmed" reports, a spokesperson said.

"We are closely monitoring the situation and are in contact with Ukrainian authorities," the spokesperson said. "Due to privacy considerations, we have no further comment."

The State Department has warned U.S. citizens against traveling to Ukraine during the war and that Russian security officials could be "singling out" U.S. citizens.

-ABC News' Benjamin Stein, Ben Gittleson and Shannon Crawford

Chairman of the Joint Chiefs of Staff, Gen. Mark Milley, said Ukrainian officials' estimate of 100 Ukrainian military deaths per day is "in the ballpark" with U.S. estimates.

Milley would not disclose exactly how many more artillery pieces the Russians have than the Ukrainians, saying that was classified, but he confirmed that they do outnumber the Ukrainians.

Milley noted that while the Russians are using large numbers of artillery to target civilian and urban areas, Ukrainians are using "much better artillery techniques" on the battlefield. Milley explained how the mortars, howitzers and HIMARS systems will give the Ukrainians a more effective combined layered system to strike at the Russians from short, medium and long distances.

-ABC News' Luis Martinez

John Kirby, joining Wednesday's White House press briefing in his new role as National Security Council coordinator, said the $1 billion in military aid announced Wednesday is the first to come from the $40 billion aid package that was passed by Congress in May.

Looking ahead, Kirby said, "you will see additional packages” coming on a “fairly routine basis.”

“We want to meter it out so that we're in lockstep with the Ukrainians and where they are on the battlefield and what they need in real time," he said.

-ABC News' Justin Ryan Gomez

The State Department has still not confirmed reports that two former U.S. service members who volunteered to help Ukrainian forces have gone missing, spokesperson Ned Price said Thursday.

The Americans, Andy Tai Ngoc Huynh and Alexander Drueke, are both from Alabama.

"We are limited in terms of what we know at the moment," Price said. "We're closely monitoring the situation we are in contact with Ukrainian authorities, as well as with the International Committee of the Red Cross, and the families of the two reported missing U.S. citizens."

The department has not "seen anything from the Russians indicating that two such individuals are in their custody," Price said.

"If the Russians were to claim that they had such individuals, I assume we would pursue that. If we had reason to believe -- credible reasons to believe -- that these individuals were in Russian custody, we would pursue that as appropriate," Price continued.

Price was asked whether the department was tracking any other cases of Americans purportedly captured on the battlefield, and he said the department was aware of reports about "one additional American whose whereabouts are unknown.”

"Similarly, our understanding was that this individual had traveled to Ukraine to take up arms," Price said. He said the person was identified as missing "in recent weeks" and that the State Department was also in contact with their family.

Alabama Rep. Terri Sewell said that Drueke’s mother reached out to her office after losing contact with him. Sewell is helping the family locate him. Alabama Rep. Robert Aderholt said his office is helping in the search for Huynh after his family reached out to the congressman's office this week.

-ABC News' Shannon Crawford