VILNIUS, Lithuania — President Biden and other world leaders announced a major security program to boost Ukraine's defenses over the long term, capping a NATO Summit in which Ukraine was not invited to join the alliance but came away with a promise of years' worth of additional military and humanitarian funding.

White House officials described the pact as a highlight of the gathering in Lithuania, saying it showcased a unified determination by the allies to protect Ukraine from another invasion like the one mounted by Russia last 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 said, flanked by other leaders of G-7 nations and Ukrainian President Volodymyr Zelenskyy. "We're going to help Ukraine build a strong capable defense across land, air and sea, which will be a force of stability in the region and deter against any and all threats."

The venue was a summit of NATO's 31 members, but the concrete commitment for an ongoing flow of aid came from a narrower group of the world's most powerful democracies: the U.S., Britain, France, Germany, Canada, Italy and Japan.

Biden and the other G-7 leaders said they would seek to deter Russia from future aggression by bolstering Ukraine's defenses and strengthening its alliances. Each country will now ink its own bilateral security arrangement with Ukraine, an effort that could invite political complexities as the war slogs through its second year.

Zelenskyy, who had blasted NATO a day earlier for not offering a timeline for accepting Ukraine as a member, took a more conciliatory tone Wednesday in thanking the group. The "Ukrainian delegation is bringing home [a] significant security victory for the Ukraine, for our country, for our people, for our children," he said.

The announcement came a day after NATO leaders declined to provide a timeline for when Ukraine would be able to join the alliance, a move that led Zelenskyy to lash out at the group in a blunt tweet. While a security pact does not provide the same kind of joint defense protections that NATO membership would have offered, it could significantly bolster Ukraine's ability to defend itself from attack.

"It's a powerful statement, a powerful statement, of our commitment to Ukraine, as it defends its freedom today, and as it rebuilds the future," Biden said. "And we're going to be there as long as that takes."

White House officials did not provide a timeline for how long it might take for the bilateral security commitments to be reached, but several G-7 countries announced other new forms of military and humanitarian support for Ukraine during the NATO summit.

Biden, who met with Zelenskyy after the announcement, has previously said he could envision a security arrangement with Ukraine similar to the kind of support the United States provides to Israel. With strong support in Congress, Washington provides more than $3.8 billion annually in military aid to Israel.

It's not clear if Biden can muster the same level of bipartisanship for funding the ongoing war in Ukraine. Some Republicans in Congress have balked at the billions of dollars U.S. taxpayers have already sent to bolster Ukraine against Russia, and House Speaker Kevin McCarthy (R-Calif.) has said Kyiv should not receive a "blank check."

Several Republican presidential candidates, who hope to oust Biden from office, have also questioned the wisdom of sending money and arms to Kyiv.

Zelenskyy, who tweeted Tuesday that "uncertainty is weakness," had hoped to leave the NATO summit with more concrete language about his country's bid to join the alliance. He said the alliance's failure to offer that would only motivate Russia to "continue its terror."

Biden's advisers contended that despite Zelenskyy's public complaint earlier in the week, the summit had been a success.

"Putin is sitting in the Kremlin thinking, 'Ah, this is good for me,'" national security adviser Jake Sullivan told a conference on the sidelines of the summit on Wednesday. "He's going to be proven wrong once again, because the alliance is going to come out of this summit united, purposeful, committed, energized."

Zelenskyy on Wednesday notably softened his message to allies in a news conference alongside NATO Secretary General Jens Stoltenberg.

"I can confirm that the results of the summit were good," he said, adding the caveat that Ukraine still wished to be invited to join NATO. "But if there had been an invitation, they would have been ideal."

"We understand that some people are afraid to speak of our membership now, because no one wants a world war. This is also understandable and logical," he said.

Asked later if he was satisfied with the results of the summit, Zelenskyy said "we have great unity for our leaders and security guarantees — that is a success for the summit."

A day earlier, the Ukrainian president jolted the summit by blasting NATO leaders' joint statement on his country's prospective membership, calling its lack of a concrete timeline "unprecedented and absurd."

His outburst angered some allies who felt they had worked hard to find a compromise on the still-divisive question of Ukrainian membership. The summit's final communiqué on Tuesday included the declaration that "Ukraine's future is in NATO" and that an official invitation would be forthcoming "when Allies agree and conditions are met."

Even as he changed his tone Wednesday, Zelenskyy continued to express some dissatisfaction with the lack of clarity over what precise conditions Ukraine would need to meet in order to become a full-fledged NATO member, taking to Twitter to declare that the "absolute majority of our people expect specifics about these conditions."

While NATO membership may not be imminent for Ukraine, a series of long-term bilateral commitments from G-7 members could be significant in convincing Russian President Vladimir Putin that he cannot prevail in a prolonged war, said Max Bergmann, a former State Department official who leads the Europe Program at the Center for Strategic and International Studies.

"It's really important that the Europeans also make that commitment, that if there is an election that goes a certain way in Washington in 2024 that European commitments will be there," he said before the summit.

The announcement follows months of discussions among G-7 leaders and Kyiv, and will also include efforts to strengthen Ukraine's economy and carry out good governance reforms, according to a White House official who spoke on the condition of anonymity to preview conversations.

Biden administration officials are expecting other countries will want to join after the announcement, the official said, adding that negotiations between Washington and Kyiv on the contours of the bilateral security package will begin soon.

Some NATO leaders suggested Zelenskyy would do well to show more appreciation for allies who are spending enormous sums and considerable political capital to help him stave off the Russian onslaught. "Whether we like it or not, people want to see gratitude," British Defense Secretary Ben Wallace told reporters, according to several British media outlets.

But others voiced sympathy for Zelenskyy's urgent appeals, and said in the end the alliance came together.

"Any of us in Zelenskyy's shoes would feel and probably act the same way," Latvian Prime Minister Krisjanis Karins said in an interview on Wednesday. "But at the end of the day, when we were all speaking before and during dinner, we were not on different pages. We were all together and the feeling inside the room was very, very positive, very collegial."

As for Wednesday's security agreement, Britain hailed the new program as a "major step toward ending the war" and suggested it would deter future acts of aggression against Ukraine.

"We can never see a repeat of what has happened in Ukraine, and this declaration reaffirms our commitment to ensure it is never left vulnerable to the kind of brutality Russia has inflicted on it again," Prime Minister Rishi Sunak said in a statement.

Japanese Prime Minister Fumio Kishida added that "the G-7 will continue to stand by Ukraine. Our solidarity will never waiver."

Biden is expected to tout the announcement as well as his broader global vision during a speech in Vilnius later Wednesday. The White House has billed the speech as the highlight of the president's five-day, three-country trip to Europe which will conclude Thursday in Helsinki.

"The president will talk about how the widespread support for Ukraine is reflective of the value of our alliances and partnerships, which he has revitalized since taking office," Amanda Sloat, National Security Council director for Europe, told reporters. "It'll be a memorable speech as the president spends the week in the middle of the world stage."

The Washington Post's David Stern in Kyiv, Ukraine, contributed to this report.

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