Jens Stoltenberg has urged NATO members to rapidly deliver more military aid to Ukraine to avoid the "grave historic mistake" of a Russian win on the battlefield.
The alliance's secretary-general said Ukrainians are "not running out of courage, they are running out of ammunition".
"Together we have the capacity to provide Ukraine with what it needs," he said. "We need to show the political will to do so."
NATO countries must "dig deep and deliver quickly" as delays will lead to "real consequences on the battlefield", Mr Stoltenberg said as he delivered the annual report for 2023.
"This is a critical moment. It will be a grave historic mistake to allow Putin to prevail," he said.
