NATO chief Jens Stoltenberg on Wednesday said there was no sign Russian President Vladimir Putin had dropped "his ambition to control the whole of Ukraine" and the war could last for years.

Stoltenberg said the conflict had reached a "critical phase" as Moscow moved its battered forces out of the north of Ukraine and rearmed them in preparation for a major offensive in the east.

"We have seen no indication that President Putin has changed his ambition to control the whole of Ukraine and also to rewrite the international order," the NATO chief said ahead of a meeting of alliance foreign ministers.

"We have to be realistic and realise that this may last for a long time, for many months, for even years. And that's the reason why we need also to be prepared for the long haul, both when it comes to supporting Ukraine, sustaining sanctions and strengthening our defences."

NATO allies have supplied Ukraine with key weaponry including anti-tank and anti-aircraft missiles that have helped its forces push back Moscow's troops.

Kyiv has demanded heavier weapons, including tanks, aircraft, air defence systems and artillery, as it girds for a reinforced onslaught in the east.

"Ukraine has a urgent need for military support," Stoltenberg said. 

"That's the reason why it is so important that NATO allies agree to further support Ukraine with many different types of military equipment, both heavier equipment but also light weapon systems."

Ukraine's foreign minister Dmytro Kuleba was due to attend the two-day meeting of foreign ministers in Brussels. 

NATO has ruled out intervening directly in the war in non-member Ukraine for fear of getting  into a direct confrontation with Russia that could spiral into a nuclear conflict. 

But it has rushed tens of thousands of additional troops to the alliance's eastern flank and is eyeing further major build-ups of forces to ward off any threats from Russia. 

"Regardless of when the war ends, this has long term implications for our security, because we have seen the brutality," Stoltenberg said. 

"We have seen the willingness by President Putin to use military force to reach his objectives."