"We must prepare for the fact that it could take years. We must not let up in supporting Ukraine," Stoltenberg was quoted as saying.
"Even if the costs are high, not only for military support, also because of rising energy and food prices." British Prime Minister Boris Johnson, who visited Kyiv on Friday with an offer of training for Ukrainian forces, also said on Saturday it was important Britain provide support for the long haul, warning of a risk of "Ukraine fatigue" as the war drags on.
In an opinion piece in London's Sunday Times, Johnson said this meant ensuring "Ukraine receives weapons, equipment, ammunition and training more rapidly than the invader".