Russia's invasion of Ukraine in February last year has galvanised the Western military alliance, set up almost 75 years ago, to face off against the Soviet Union.

But members of the military bloc are split over Ukraine, with NATO chief Jens Stoltenberg saying all members agree to stick by a 2008 pledge that Ukraine will become a member at some undefined point.

"We are reasonable people and we understand that we are not going to drag a single NATO country into a war," Zelensky said during a briefing along Estonian President Alar Karis.

"Therefore, we understand that we will not be members of NATO while this war is going on. Not because we don't want to, but because it's impossible," Zelensky added.

Ukraine is a candidate to join both NATO and the European Union but some European capitals are wary of setting a formal timeline for membership as Russia's invasion continues.

Joining NATO would mean Ukraine would be covered by the alliance's Article 5 collective defence clause that obliges all members to help defend it if attacked.

Friday's comments were a rare admission by the Ukrainian president, who has stepped up pressure on NATO and the European Union to open their doors to Ukraine since the beginning of Russia's invasion in February, 2022.

On Thursday, Zelensky told a summit in Moldova that any doubts European leaders show before admitting Kyiv into the NATO alliance will embolden Russia to attack more countries.

An option being weighed is major powers offering Ukraine bilateral security assurances in the years before it becomes a full NATO member.