Kyiv has long pushed for heavier weaponry, including tanks, but Western nations have been reluctant, citing fears of becoming drawn into the war or provoking Russia

"The recent pledges for heavy warfare equipment are important -- and I expect more in the near future," Stoltenberg told the Handelsblatt daily.

The comments come ahead of a meeting this week of the Ukraine Defence Contact Group, which coordinates arms supplies to Kyiv, at Ramstein Air Base in Germany.

"We are in a decisive phase of the war," Stoltenberg said. "Therefore, it is important that we provide Ukraine with the weapons it needs to win."

Since the invasion, the Ukrainians have built momentum and Western nations have been expanding the range of weapons being provided.

Earlier this month, France, Germany and the United States respectively promised French AMX-10 RC light tanks, 40 German Marder infantry vehicles, and 50 Bradley fighting vehicles.

However, pressure is growing on the allies to go further and agree to the delivery of battle tanks.

British Prime Minister Rishi Sunak on Saturday pledged to provide 14 Challenger 2 tanks to Ukraine, making it the first Western country to supply the heavy tanks Kyiv has been calling for.

Stoltenberg said Russian President Vladimir Putin had made a mistake by attacking Ukraine.

"He overestimated the strength of his own armed forces. We see their missteps, their lack of morale, the leadership problems, the poor equipment," he said.

But the Russians "have demonstrated that they are prepared to take heavy losses to achieve their goals".