Moscow on Wednesday warned of a "serious" escalation of the Ukraine crisis if London gives Kyiv the armour piercing rounds.

"NATO allies are following international rules and international law in everything they do in their support for Ukraine," Stoltenberg told AFP when asked about the British plans and Russian complaints.

"The dangerous thing is the war, which is taking thousands of lives," he said at the operational launch of a new fleet of NATO-EU air-refuelling planes at a Dutch airbase.

"The most important thing that can be done to reduce risks is for President Putin to stop the war."

British junior defence minister Annabel Goldie confirmed on Monday that the UK would provide Ukraine with the depleted uranium rounds. The heaviness of the metal allows shells to more easily penetrate steel.

- 'Difficult situation' -

Russian Foreign Minister Sergei Lavrov said that their use would be a "step towards a further escalation, and a serious one at that".

Lavrov added that it would "sharply reduce" Ukraine's ability to "produce high-quality, uncontaminated food".

The United States on Wednesday dismissed what it called Moscow's "strawman" argument.

The NATO secretary general meanwhile said there was still a "difficult situation" around the frontline town of Bakhmut, where Kyiv on Thursday threatened an impending counterattack.

"Wars are by nature unpredictable. What we see in Bakhmut is heavy fighting and (a) difficult situation," said Stoltenberg.

But he said NATO equipment and training for Ukraine was "enabling them to make advances and new gains".