Stoltenberg said that when Russia launched its invasion, its military was seen as the second strongest in the world, adding: "Now the Russian army is the second strongest in Ukraine.

"The Ukrainians are gradually gaining ground and it proves the importance of our support and also our ability and willingness to continue the support," he told EU lawmakers.

"This is heavy fighting, difficult fighting, but they have been able to breach the defensive lines of the Russian forces, and they are moving forward."

In recent days, Ukraine has claimed to have broken through Russia's heavily fortified first line of defence in its gruelling southern offensive.

Ukraine received shipments of Western armour from NATO members, including battle tanks and armoured vehicles, to aid its push.

But Kyiv admits the months-long drive southwards has been slower than hoped and has inflicted a heavy toll on its forces.

Ukraine's troops have reportedly had to increasingly resort to smaller-scale attacks on foot after full-frontal assaults were stalled by the dense minefields laid by the Russians.

"Hardly any time in history we have seen more mines on the battlefield than we see in Ukraine today. So it was obvious that this was going to be extremely difficult," Stoltenberg said.

"But the Ukrainians decided to launch the offensive because they are going to liberate their land, and they are making progress. Not perhaps as much as we hoped for, but they are gaining ground gradually, some 100 metres per day," he said.

"In hardly any war we will see only victories for the side we support. There will be bad days and good days. We need to be with Ukraine not only (in) good times, but also bad times."

He insisted "we support them when they win and if they lose, we are there with Ukraine."

Russia's invasion of Ukraine has reinvigorated the US-led NATO military alliance, leading it to bolster its forces on its eastern flank and grow its membership.

Stoltenberg said he expected Turkey to approve Sweden's bid to join "as soon as possible" after its parliament reconvenes.

Turkish President Recep Tayyip Erdogan agreed at the NATO summit in Vilnius this summer to clear the way for Stockholm's membership after a year of holding it up.