"Of course we would like them to liberate as much territory as possible, as quickly as possible, but even though the frontline has not moved, Ukrainians have been able to inflict heavy losses on the Russian invaders," Stoltenberg said.

The assertion from the head of the Western military alliance comes after Ukraine's top general said the conflict with Russia has ground to a "stalemate".

"We see high casualty numbers, and some of the most intense fighting that we have seen in the whole war, have actually taken place over the last weeks and couple of months," Stoltenberg said.

"We need to distinguish between the fact that the frontline is not moving so much, and the fact that actually there's very heavy fighting going on."

The NATO chief insisted that, while Ukrainian forces were not currently progressing, Kyiv had already recaptured 50 percent of territory seized by Russia since its full-scale invasion.

"This is a big win for Ukraine," he said.

"Meanwhile, Russia is weaker politically, economically and militarily."

Ukraine's failure to score a decisive breakthrough comes as doubts have emerged over the future of military support from key backer the United States.

Stoltenberg pointed to recent pledges of military assistance from Germany and the Netherlands worth 10 billion euros ($11 billion) as evidence the West's support was not drying up.

Stoltenberg played down fears that any dwindling of weapons deliveries to Ukraine would force Kyiv to the negotiating table with Russian President Vladimir Putin before it was ready.

"It is for Ukraine to decide what are acceptable ways to end this war," Stoltenberg said.

"Our responsibility is to support Ukraine and to enable them to liberate as much land as possible and to put them in the best possible place when or if negotiations may start."

NATO foreign ministers meeting in Brussels on Tuesday will look to reassure Kyiv of their backing despite Israel's war with Hamas diverting much attention.

Washington insists it is supporting Ukraine "for the long haul" despite opposition but hardline Republicans are so far blocking major new commitments.

Stoltenberg said NATO allies had sent increasingly advanced weapons systems to Ukraine to help it advance and work was going on to try to better link up how Kyiv's forces use technology like drones and cyber to attack.

"I'm absolutely certain that the message from NATO allies at this meeting... is that we need to stand by Ukraine," Stoltenberg said.

"It is our security interest that President Putin does not win this war."