For Russia, the bloc has deliberately encroached on Moscow's doorstep and emerged as the most serious external military threat in decades.

Preventing Kyiv from joining was one reason President Vladimir Putin gave for Russia's all-out assault on Ukraine in February 2022.

And if Ukraine is put on the path to NATO membership at the bloc's summit this week, it would realise a nightmare for Moscow -- Ukraine, a once "brotherly nation," joining the alliance.

"The deployment of NATO military infrastructure close to Russia's borders is seen in Moscow as a real threat for the security of the country," Georgy Bovt, a political expert, told AFP.

"Yesterday they bombed Belgrade, tomorrow they'll bomb Smolensk," said the expert, characterising Russia's official thinking.

NATO strikes on Belgrade in 1999 during the Kosovo War shocked Russia and became a turning point in ties with the alliance which had started on a better footing.

- 'It's frightening' -

After the end of the Cold War, Russia in 1994 joined the Partnership for Peace programme, an initiative aimed at fostering cooperation between NATO and countries in the former Soviet bloc.

Almost three decades later, the alliance has deployed military contingents in eight countries in Central and Eastern Europe, four of which border Russia.

Russia also says that NATO leaders betrayed a promise not to expand the body after the fall of the Soviet Union -- a mantra frequently repeated by Russian President Vladimir Putin for years.

Following Lithuanian President Gitanas Nauseda's call this month for NATO to establish permanent military bases near Russia's border, Kremlin spokesman Dmitry Peskov said that "Europeans do not understand the mistake they are making".

The Kremlin has said it will be "carefully monitoring" the Vilnius summit.

In central Moscow 22-year-old English teacher Anastasia Galantseva echoed concerns from the Kremlin.

"It's frightening," she told AFP, referring to NATO's expansion.

"And it's honestly not known how events will develop further. But I want to believe that it will end peacefully," she said.

Even though Ukraine is unlikely to be invited to join NATO, the summit "will be another step towards Ukraine's NATO membership," Bovt said.

As a result, the expert said that Russia now will "no longer have any reason to put an end to its military operations" aimed at preventing such a scenario.

"And, viceversa, as long as the war continues, Ukraine will not be accepted into NATO," he said.

- 'Colourful spectacle' -

Sweden instead is due to join imminently after the last hold-outs, Hungary and Turkey, gave their consent to membership despite their close ties with Moscow.

It will follow Finland, which joined in April, in a move doubling NATO's border with Russia to 2,500 kilometres.

The decision to allow in the new members risks plunging relations between Moscow and Ankara, even more so after Ukrainian President Volodymyr Zelensky returned from a visit to Turkey over the weekend with five Ukrainian military commanders.

The Kremlin reacted angrily since the men had been due to remain in Turkey for the duration of the conflict under the terms of a prisoner exchange deal with Russia.

Kremlin spokesman Dmitry Peskov said Tuesday that Ankara would be "more oriented towards the West than towards Russia" and called on Turkish authorities to "take off the rose-tinted spectacles" because "nobody wants Turkey in Europe".

Even though tensions are growing, the fact that Zelensky has not received any formal invitation to join NATO provides at least some reassurance for Russia -- and is even a subject of mockery.

Russian foreign ministry spokeswoman Maria Zakharova on Tuesday joked that the summit as "a very colourful spectacle".

She said she expected NATO to perform "cello solos in support of Ukraine" but offer no invitation at the end.