Zelensky was speaking during a visit to Kyiv by NATO Secretary General Jens Stoltenberg -- his first since the start of Russia's invasion in February 2022.

Russia sees Ukraine's NATO membership bid as an existential threat, while Ukraine says the invasion justifies even more its years-long campaign to join the alliance.

Zelensky said a NATO summit in July "could become historic" if Ukraine received a formal invitation to join.

"It is time to take the appropriate decision," Zelensky said, speaking at a press conference with Stoltenberg.

He also asked for NATO's help to "overcome the reluctance" of some member states in providing long-range rockets, modern fighter jets and armoured vehicles.

NATO members have sent some Soviet-era fighter jets to Ukraine, but no modern planes such as the US-designed F16 have been pledged despite Ukraine's requests.

Ukraine's Western supporters have also been reluctant to send long-range rockets because of concerns that Ukraine could use them to hit targets within Russia.

Stoltenberg said the alliance would "ensure that Ukraine prevails" against Russia, but did not hold out any immediate prospect of alliance membership.

"Ukraine's future is in NATO, all allies agree on that. At the same time, the main focus of the alliance, or allies now, is to ensure that Ukraine prevails," he said.

- Vilnius summit -

He said the issue of membership would be "high on the agenda" at the NATO summit due to take place in July in Vilnius.

"NATO stands with you today, tomorrow, and for as long as it takes," he said.

Stoltenberg also said that it was important that arms already sent to Ukraine "work as they should", including having sufficient ammunition and repairs.

Stoltenberg's visit to Ukraine was not announced ahead of time -- in line with the policy followed by many Western officials for security reasons.

It comes 14 months into Russia's invasion and ahead of an expected Ukrainian counter-offensive.

Russia says NATO's deliveries of military aid to Kyiv are proof that the alliance and the United States are waging a war by proxy in Ukraine.

The Kremlin said on Thursday that preventing Ukraine joining the Western military alliance remains one of the main aims of its invasion.

"Otherwise, it will pose a serious danger to our country, to its security," the Kremlin said.

Russian President Vladimir Putin has claimed he had no choice but to send troops into Ukraine to stop it moving into the NATO fold.

But Western diplomats say membership for Ukraine remains a distant prospect.

NATO members are bound by the principle of collective defence in which an attack on one is considered an attack on all.