"In recent months, NATO allies have not provided the support we have promised," he said in a speech on a visit to Berlin.

"But it is not too late for Ukraine to prevail, because more support is on the way."

Ukraine has suffered setbacks on the battlefield against Russia's forces due to ammunition and weapons shortages.

But Stoltenberg noted that the US Congress had finally approved a bill to provide billions of dollars in support for Ukraine, and other countries including Britain, Germany and the Netherlands had made new pledges.