Ukraine is increasingly pessimistic about taking a significant step forward in joining Nato as leaders of the western military alliance are set to assemble on Tuesday in the Lithuanian capital, Vilnius.

Kyiv is expected to be offered a package of last-minute “enabling security guarantees” at the two day summit – an assurance from countries such as the US, UK, France and Germany that military aid and training will continue in the long term.

The US and Germany, in particular, appear unwilling to support Ukraine joining the 31-country alliance while the conflict with Russia is ongoing, leaving those close to Ukraine’s president, Volodymyr Zelenskiy, not hopeful of obtaining a concrete pathway to Membership.

Dmytro Kuleba, Ukraine’s foreign secretary, acknowledged on Saturday that “what is missing” is a decision to make the country’s path to Nato membership even faster with “clarity on Ukraine’s accession invitation”. Vilnius, he suggested, would be “a unique moment” to bring this forward.

The US president, Joe Biden, on Sunday told CNN that Ukraine was “not yet ready” for Nato membership. “Nato is a process that takes some time to meet all the qualifications – from democratisation to a whole range of other issues,” he said, adding that Nato needed to “lay out a rational path” for membership.

He suggested the US could provide military aid similar to the support it has long provided to Israel.

Kyiv believes Nato membership, carrying with it the defensive prospect of the western nuclear umbrella, is the only realistic long-term guarantee of its security since it is still likely to face a hostile Russia even if it can somehow force Moscow to sue for peace.

Lingering in the background is the Bucharest summit in 2008, when a then enthusiastic US was talked out of offering Ukraine and Georgia a path to Nato membership, known as a membership action plan. Instead, the two countries were simply told they could become members at some indeterminate point in the future.

Georgia was attacked by Vladimir Putin’s Russia that year, and Ukraine was first invaded in 2014 as the promise of eventual membership proved to be no deterrent, whilst full alliance members – including the three Baltic states, Lithuania, Latvia and Estonia – remaining unthreatened by direct military aggression from Moscow.

Politically, too, there is a firm recognition that Ukraine should be allowed to become a member at some point; there will be “a reaffirmation that Ukraine will become a member of Nato”, the secretary general, Jens Stoltenberg, said on Friday. But even such a simple commitment hands Moscow an incentive to keep on fighting.

“If Nato promises Ukraine membership when the war ends, that in a way allows Russia to procrastinate in the war,” said Orysia Lutsevych, a Ukraine expert with the Chatham House thinktank. Alternative wording, suggesting Ukraine could join when the security situation allows, might help, she added, but Nato members understand the commitment it brings, meaning it remains “a difficult balance to strike”.

At the heart of Nato is its founding charter’s article 5, which states each member agrees “that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all”. It requires member states to join in defending the country under attack.

But the White House, expecting to face the Putin-friendly Donald Trump in next year’s inevitably protracted election battle, has other calculations. “There is concern about the appetite of the American public for getting involved in the conflict in Europe,” said Alice Billon-Galland, a research fellow with Chatham House.

That means there will be heavy emphasis on other promises to Ukraine at Vilnius. At the heart of these are possibly fresh guarantees, emphasising long-term weapon supplies and other forms of support, including economic, by a self-selecting group of Nato members including the US, UK, France and Germany.

Last-minute discussions about the package will probably form part of Biden’s stopover in London on Monday, where he will meet the UK prime minister, Rishi Sunak, before they go separately to the Vilnius meeting, which starts Tuesday lunchtime.

The theory takes Ukraine down the path of the so-called “porcupine strategy” – where the country becomes highly militarised and, like Israel, hard to defeat. On Friday, French officials described these as “enabling security guarantees” designed to show Putin that key countries in the west will support Ukraine “for as long as it takes” – allowing Kyiv to sustain a long war.

But one former official warned of “repeating the great Bucharest fudge” and so prolonging the war. John Foreman, Britain’s defence attache to Moscow until last year, said he believed the US president had “misread Russian intent and overplays the risk of a widening of the conflict. I don’t believe Russia has either the capability or intent to do this. Leaving Ukraine neither in nor out of Nato will embolden Russia.”