Around 50 alliance members and partners are meeting in Brussels to discuss Ukraine's desperate pleas for more hardware as it struggles to hold back Russia's onslaught in the east of the country.

"Ukraine is really in a very critical situation and therefore, it's an urgent need to step up," Stoltenberg told journalists ahead of a gathering of NATO ministers.

The West has poured major supplies of arms into Ukraine to help it fight the Kremlin but Kyiv complains it has only received a fraction of what it needs and is clamouring for heavier weaponry.

Stoltenberg said that the allies had moved from sending older equipment to delivering "more long range, more advanced air defence systems, more advanced artillery, more heavy weapons".

"So it is also a fact that when we now are actually starting the transition from Soviet-era weapons to more modern NATO weapons there will also be some time needed to just make the Ukrainians ready to use and operate these systems," he said.

He said NATO members, such as the Netherlands, are urgently looking to offer training to Ukrainian forces to get them up to speed on the new heavy guns going in.

Stoltenberg said alliance leaders at a summit in Madrid later this month should agree a "comprehensive assistance package" for Ukraine to help switch its forces to NATO-standard weapons over the longer term.

Russia's invasion of its pro-Western neighbour has shaken the US-led military bloc and forced it to rush forces to eastern Europe.

The NATO summit is set to sign off on a new strategy aimed at bolstering the alliance's ability to deter an attack by Moscow on its members.

Stoltenberg said he expected to see allies commit to having more high-readiness troops dedicated and trained to defend the eastern flank and more weaponry in place in the region.

But he said the soldiers would likely not be based permanently in the eastern allies -- dashing hopes of countries like the Baltic nations which had eyed major NATO deployments to their territories.