While NATO allies had made "encouraging" recent commitments on military support, he said: "We should have given them more earlier," APA reports citing AFP.
He was speaking to reporters ahead of a working session on Ukraine, also attended by Ukrainian Foreign Minister Dmytro Kuleba.
Ukraine has struggled on the battlefield for months, outgunned and outnumbered by Russian forces amid a shortage of Western military aid.
Stoltenberg said there had been "important, encouraging signs" of new military support from NATO allies, including the scheduling of a US Congressional vote this weekend on a long-stalled $61-billion package of aid.
"It's of vital importance that NATO allies sustain and step up their support. We will focus today on air defence. There's an urgent, critical need for more air defence," Stoltenberg said.
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