KYIV, Ukraine - Top Ukrainian officials are hoping that next week's NATO leaders' summit in Vilnius, Lithuania, will be an epic moment - when Ukraine finally receives a "clear signal" that it will eventually join the alliance, anchoring the country in the West's security infrastructure and sending an unequivocal message to Moscow.

Yuriy Sak, an adviser to Ukrainian Defense Minister Oleksii Reznikov, said the summit "must end" with Ukrainian President Volodymyr Zelensky and NATO Secretary General Jens Stoltenberg "standing next to each other" and proclaiming, in Sak's words: "Today, we have reached a historical decision. Today, we have invited Ukraine to join NATO."

"Then everyone drinks champagne," Sak said.

But just days before leaders arrive in the Lithuanian capital, it's far from clear that corks will be popping - or that there are even any bottles to put on ice. Instead, questions loom about what options Ukraine will be left with if its hopes are dashed, which may probably be the case.

NATO allies are still negotiating what exactly to offer Ukraine at the meeting, which begins Tuesday.

On Friday, NATO Secretary General Jens Stoltenberg kept things vague, saying only that allies would "reaffirm that Ukraine will become a member" - which has been the alliance's fraught position for 15 years - and "unite on how to bring Ukraine closer to this goal."

Washington, which holds the greatest sway over decisions of the 31-member alliance, has been maneuvering for months to lower Kyiv's expectations by focusing the conversation on "security guarantees" rather than membership in the near term, which many allies see as impossible to even discuss so long as Ukraine remains at war with Russia.

Reznikov said the Vilnius summit will afford the alliance an opportunity to "correct the mistake" that was made at a 2008 NATO summit in Bucharest - where Ukraine and Georgia were told they would become members sometime in the future, without saying when or how this would come about.

In hindsight, many officials and analysts say this made the countries a target for Russian invasion - Georgia in 2008, and Ukraine in 2014 and again last year - without providing the protections of NATO's collective defense doctrine, in which an attack on one ally is considered an attack on all.

"'The doors are open,' they told us, but they didn't show us where to find these doors, how to get in - and we're ready," Reznikov said in an interview.

Others, however, still question Kyiv's "readiness." Ukraine still has a long way to reform its military and tackle its chronic problem with corruption, which will be a concern of Western politicians when looking at Kyiv's application.

Publicly, Ukrainian officials are pushing hard for an invitation even without a fixed date. In an interview broadcast on CNN Wednesday evening, Zelensky called on President Biden to invite Ukraine into the alliance "now."

Biden, Zelensky said, was NATO's chief "decision-maker."

Zelensky said he understood that membership could not happen while Ukraine is fighting Russia's invasion, in keeping with the alliance's policy of requiring territorial disputes to be resolved before accession. "We understand everything," Zelensky said. "But this signal is really very important."

With or without membership, Ukrainian officials are looking for security commitments by Western nations "without delay and as soon as possible," which would potentially encourage Moscow to withdraw its forces. Many analysts say Russian President Vladimir Putin is counting on Ukraine's Western supporters to grow exhausted and halt the expensive flow of weapons and economic aid they have been sending to Kyiv.

Such security guarantees could also serve to deter Russia from any major acts of aggression in the future. "I am sure that if the regime in the Kremlin does not change in the coming years, even after our victory, there will be - in their heads - a desire for revenge," Reznikov said.

Details of the security package are still not finalized, diplomats said. But U.S. and NATO officials have described evolving proposals for bilateral or multilateral agreements that they characterized as mutual defense pacts or security memorandums with Ukraine.

The Biden administration has tried to shift the debate toward long-term security pacts as an alternative to near-term membership. The United States also has given tentative backing to a plan to remove barriers to Ukraine's entry - by, say, allowing Ukraine to later circumvent the alliance's Membership Action Plan in the future, but without actually setting a timeline.

It is unclear, however, that such agreements would do much more to help Ukraine at the moment. No NATO ally appears willing to send its own soldiers to fight in Ukraine. And the United States has repeatedly balked at sending Ukraine its most advanced weapons.

Reznikov, in a separate text message, said that "we've made it very clear to our partners" that security guarantees for Ukraine "should be comprehensive and include military and financial assistance, as well as economic guarantees."

And they "have to be real," he added - unlike the Budapest Memorandum, which Ukraine signed in 1994, that gave assurances that Russia and other powers would not use military force against Ukraine in exchange for Kyiv giving up its nuclear weapons. This, Reznikov said, "turned out not worth the paper it was written on."

Ukraine may find it difficult to fulfill its security needs, however, leaving it in an uncertain situation similar to the one it found itself in before the war with Russia.

Virtually any outcome of the Vilnius summit, short of an immediate invitation to join NATO, seems likely to leave Ukraine in pretty much the same limbo that it faced after Bucharest. While Kyiv has received an unprecedented supply of weapons and ammunition from the West, it is not clear that Ukraine's allies will be able to sustain this indefinitely. Democratic Western governments must also adjust to election results if voters demand change.

Without a clear commitment, some Ukrainian officials see a worst-case scenario in which support for Kyiv eventually crumbles, especially if Biden is defeated by a Republican in next year's presidential race. Ukraine and its supporters hope to lock in its security assurances for years to come, regardless of who occupies the White House.

While there is broad agreement that the alliance should enhance its political relationship with Kyiv, there are big divisions about what that actually means.

In the days leading up to the summit, as Ukraine fights Russia, NATO country aides are engaged in their own battle over how to word the summit's concluding declaration.

NATO is expected to upgrade the NATO-Ukraine Commission, a forum for consultation, to a Ukraine-NATO Council, a shift that NATO insists will give Ukraine more agency, including the ability to convene meetings and raise issues. The council's first meeting will be held Thursday, the second day of the summit, with allied leaders in attendance.

Beyond that, things are very much in flux.

Aside from the creation of the council, Reznikov said Ukrainian officials have a list of key areas of cooperation with NATO - in procurement, training and joint defense planning - which they hope the Vilnius summit will confirm.

In the Vilnius declaration, allies must find language that does not give Russia a "veto" over membership, said Camille Grand, a former NATO assistant secretary general for defense investment who is now a policy fellow at the European Council on Foreign Relations. "We have to counter the notion that to be eligible for membership they have to be at peace with Russia," Grand said, "that if you have a frozen conflict, you are not welcome."

Reznikov and other Ukrainian officials say Ukraine has already more than proved its worth. NATO, he said, was created "as a solution against Soviet aggression" - a role that Moscow has inherited. "We are carrying out the NATO mission that it was created for - the only army in the world doing this," Reznikov said.

"There is no other such army with such experience to defeat Russia," he said. "Other arguments are not even necessary."

If Kyiv does not receive an invitation in Vilnius, or some clear commitment, the disappointment will be felt across Ukraine, civil society activists say.

A formal invitation would send "a clear signal to Russia that Ukraine is not considered as a buffer zone anymore," said Hanna Hopko, a former member of Ukraine's parliament and founder of the International Center for Ukrainian Victory, an advocacy group.

"The crime of inaction is worse than crimes of aggression when you know you can help and save lives but don't do this or deliberately delay these decisions," Hopko said. "This is how evil prevails."

Rauhala reported from Brussels.