Stoltenberg was in Ukraine when Russia claimed the capture of two eastern villages in 24 hours, rushing an advance in the Donetsk region before crucial US weapons reach Kyiv's exhausted forces on the front.

"Ukraine has been outgunned for months, forced to ration its ammunition... But it's not too late for Ukraine to prevail," the NATO secretary general said at a press conference with Zelensky.

He said months-long delays in US military aid to Ukraine had "serious consequences on the battlefield".

Stoltenberg said that "more support is on the way".

"Our allies are looking into what more they can do and I expect new announcements soon. So we are working hard to meet Ukraine's urgent needs," he promised.

Ukraine has been dependent on Western military aid to counter Russia's larger and more powerful army.

Zelensky urged faster deliveries to bolster the front line.

"The Russian army is now trying to take advantage of the situation when we are expecting supplies from our partner," the Ukrainian leader said.

"That is why promptness in supply literally means stabilisation of the front line," he said.

"Together we must disrupt the Russian offensive," he said.

Speaking of the long-awaited aid, Zelensky said "some things have started to arrive" but declined to "go into details".

US President Joe Biden gave final approval for the aid -- blocked in Congress for months -- last week, saying shipments will start "right away".

On Sunday, Ukraine said the situation was deteriorating for its troops in the eastern Donetsk region.