NATO foreign ministers meeting in Oslo Thursday will try to narrow divisions on Ukraine's push to join, upping spending and finding a new alliance chief, ahead of a July summit.

Russia's war on Ukraine has galvanised the Western military alliance set up almost 75 years ago to face off against the Soviet Union.

Chief among them is Kyiv's push to join NATO, an organisation that requires consensus to take decisions.

"There will be some challenging discussions among allies in the run-up to Vilnius, including on security guarantees or assurances for Ukraine and their desire for NATO membership," alliance Secretary General Jens Stoltenberg said Tuesday.

"I cannot anticipate the outcome of the discussions, but what is clear is that all NATO allies agree that NATO's door is open."

Ukraine's President Volodymyr Zelensky, backed by NATO countries in eastern Europe, is calling for a "clear message" at the July summit that Kyiv will join once the conflict with Russia ends.

Ukraine concedes it will not become a member while fighting rages on its territory. But it wants the alliance to move beyond a vague 2008 pledge that it will one day be in NATO.

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

Despite the divisions, diplomats at NATO remain relaxed that a compromise can be found in the intense negotiations planned before the Vilnius summit.

One option is countries offering Ukraine bilateral security assurances outside the alliance's framework.

France has said it could be ready to provide some form of guarantee.

But there are major questions how any commitments to Ukraine could work.

On a practical level, Stoltenberg is pushing for a decade-long programme worth 500 million euros ($530 million) per year to help Ukraine's military switch to Western standards.

That would be on top of the tens of billions of dollars in arms that allies have already sent.