"I think what we saw yesterday (Monday) is actually a sign of weakness, because the reality is that they are not able to make progress on the battlefield. Russia is actually losing on the battlefield," Stoltenberg said on the eve of a meeting of NATO defence ministers.

"So the way they're able to respond is by indiscriminate attacks on Ukrainian cities, hitting civilians, critical infrastructure."

Russia has unleashed a wave of missiles strikes across Ukraine after a blast damaged a key bridge to the annexed Crimea peninsula that the Kremlin blamed on Kyiv.

Western backers of Ukraine will meet in Brussels on Wednesday to discuss further arms deliveries to Kyiv to help defend itself and "liberate territory from Russian occupation," Stoltenberg said.

He welcomed announcements from the US and Germany that advanced air defence systems were on their way and said he looked forward to "further deliveries".

NATO allies have funnelled weaponry worth billions of dollars to Ukraine for it to battle Moscow's invasion and are now scrambling to resupply their own dwindling stockpiles.

"We will take decisions to increase our stockpiles of munitions and equipment, to speed up the delivery of capabilities," Stoltenberg said.

NATO ministers will hold a meeting of their regular nuclear planning group after Russian President Vladimir Putin ratcheted up tensions with veiled threats of using nuclear weapons.

"We have not seen any changes in Russia's posture but we remain vigilant," the NATO chief said.

NATO is set to hold its annual nuclear deterrence exercise, named Steadfast Noon, next week. Stoltenberg called it "routine training".

The meeting in Brussels this week will also focus on bolstering protection for critical infrastructure after suspicious leaks in the Nord Stream gas pipelines running from Russia to Europe that NATO branded "sabotage".

Stoltenberg said the alliance had "doubled our presence in the Baltic and North seas to over 30 ships supported by maritime patrol aircraft and undersea capabilities".

Sweden and Denmark are currently probing the leaks. NATO has not formally apportioned blame for the incident.