VILNIUS, Lithuania — White House national security adviser Jake Sullivan rebutted criticism Wednesday from Ukrainian President Volodymyr Zelensky for not extending NATO membership to the war-torn nation — saying the US “deserves a degree of gratitude” for billions it has provided for Ukraine’s defense.

“I think the American people do deserve a degree of gratitude from the United States government for their willingness to step up and from the rest of the world as well,” Sullivan said at a public forum during the NATO summit in Lithuania’s capital, shortly before President Biden himself met with Zelensky.

Although Sullivan did not directly criticize Zelensky, the implication was clear after the Ukrainian leader blasted America’s stance on NATO expansion Tuesday.

Zelensky adopted a more conciliatory tone Wednesday afternoon while meeting with Biden — thanking the US for providing billions in arms to his government and hailing Biden’s “great idea” of G7 nations making commitments to his country.

“Thank you for acknowledging the American people,” Biden replied. “I think we’ve given more than every other nation combined.”

Sullivan’s thinly veiled criticism of Zelensky was in response to a Ukrainian activist’s question about whether the US was “afraid of Russia” in stopping short of meeting all of Kyiv’s requests, which include a demand for an invitation to NATO.

Zelensky on Tuesday issued a fiery tweet rebuking NATO leaders for preparing an “unprecedented and absurd” new document describing how his country would not be permitted to join the military alliance until member nations agree that certain conditions are met.

“It seems there is no readiness neither to invite Ukraine to NATO nor to make it a member of the Alliance,” Zelensky fumed on Twitter as he traveled to Vilnius. “This means that a window of opportunity is being left to bargain Ukraine’s membership in NATO in negotiations with Russia. And for Russia, this means motivation to continue its terror. Uncertainty is weakness. And I will openly discuss this at the summit.”

Zelensky offered a more measured assessment Wednesday morning, saying at a press conference alongside NATO Secretary-General Jens Stoltenberg that it was “understandable” that NATO leaders would fear sparking a “world war” with Russia by admitting Ukraine now, but adding that an invitation would be an important “signal” to Moscow.

Sullivan’s sharp words were a rare airing of criticism from the Biden administration, which has gradually met many of Zelensky’s demands for advanced weaponry.

“The United States of America has stepped up to provide an enormous amount of capacity to help ensure that Ukraine’s brave soldiers have the ammunition, air defense, the infantry, fighting vehicles, the mine clearing equipment and so much else to be able to effectively defend against Russia’s onslaught and to take territory back as well,” Sullivan also said.

“There has been a lot of conspiracy theorizing that simply is not based on any reality whatsoever,” he added.

Congress has approved more than $113 billion in aid for Ukraine since Russia launched its invasion in February of last year.

In remarks capping his stay in Vilnius Wednesday, Biden credited NATO with warning Ukrainians that Russian President Vladimir Putin was going to invade and said the alliance was preparing “to deter any aggression against any member state.”

“We pursued intense diplomacy with Russia seeking to avert this terrible war. And when Russia bombs began to fall, we did not hesitate to act,” Biden said. “We rallied the world to support the brave people in Ukraine as they defend their liberty and their sovereignty with incredible dignity. 

He also boasted of America’s leadership in creating the Ukraine Defense Contact Group, which brings together defense leaders from more than 50 nations for monthly meetings to coordinate all international military aid for Kyiv to “defend itself both now and … in the future, as well.”

“Putin still doubts our staying power,” Biden said. “He’s still making a bad bet that the conviction and the unity among the United States and our allies and partners will break down.

“He still doesn’t understand that our commitment, our values, our freedom is something he can never, never ever, ever walk away from,” he added, calling NATO “stronger, more energized and more united.”