NATO's support for Kyiv in the Russia-Ukraine war has made a "huge difference," allowing it to advance and regain some territory, the bloc's chief Jens Stoltenberg said Friday.

"The war in Ukraine is entering in critical phase" because Ukrainian forces managed to stall Russia's offensive in the Donbass region and retake territory in the south and east of the country, Stoltenberg told a joint news conference with U.S. Secretary of State Antony Blinken after their meeting.

Stoltenberg warned that Ukraine and NATO must "be prepared for the long haul" but urged allies to continue to provide support for Ukraine because "we see that this is making a huge difference on the ground."

"If Ukraine stops fighting, it will cease to exist as an independent nation, so we must stay on course for Ukraine's sake and for ours," he said.

For his part, Blinken said he is "absolutely confident" that Ukraine will win the war because "people are fighting for their homeland, fighting for their future."

He explained that during his visit to Kyiv on Thursday, he informed Ukrainian President Volodymyr Zelenskyy that the U.S. would provide an additional $2.8 billion in security assistance to his country and its neighbors, including $675 million in new military aid to Ukraine.

As part of the broader response to Russia's aggression against Ukraine, NATO leaders agreed in June on the biggest overhaul of NATO's collective defense and deterrence since the Cold War.

The reforms include a new deployment model with earmarked forces pre-assigned to defend specific NATO countries, 300,000 troops at high readiness, and more pre-positioned equipment and weapon stockpiles in the eastern flanks.