Nearly four months after Russian forces invaded Ukraine, NATO is now warning that the war could drag on for years. 

"Nobody knows. We must prepare for the fact that it could take years. We must not stop supporting Ukraine. Even if the costs are high, not only for military support but also because of rising energy and food prices," NATO Secretary General Jens Stoltenberg told the German newspaper Bild am Sonntag. 

"If Putin learns from this war that he can simply carry on as he did after the 2008 war in Georgia and the occupation of Crimea in 2014, then we will pay a much higher price," he added, referencing food and gas prices that have surged in the wake of the war. 

Some top U.S. officials predicted just weeks before the invasion that Ukraine's capital of Kyiv would fall in just 72 hours once Russia invaded, but Ukrainian forces have put up fierce resistance, aided by tens of billions of dollars worth of defense and military aid from the West. 

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Stoltenberg said that a new package of arms is on the way for Ukraine pending a NATO summit in Madrid later this month. 

"The NATO allies are supporting Ukraine financially, with humanitarian aid and, last but not least, with military equipment. In addition, the NATO summit - to which I have invited President [Zelenskyy] - will adopt a comprehensive aid package for Ukraine. For now, it's about helping Ukraine fight for its existence against Putin's brutal invasion," Stoltenberg told the newspaper. 

After Russian forces pulled back from Kyiv in April, fighting has coalesced around the eastern Donbas and Luhansk regions, where Ukrainian forces are trying to hold off an offensive for the industrial city of Sievierodonetsk. 

"With more modern weapons, the probability increases that Ukraine will be able to drive Putin's troops out of the [Donbas] again," Stoltenberg told Bild am Sonntag.

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Zelenskyy visited frontline forces in the south on Saturday, where Ukraine is defending key Black Sea port cities like Mykolaiv and Odessa. 

"It is important that you are alive. As long as you live, there is a strong Ukrainian wall that protects our country," Zelenskyy told soldiers in Odesa as he handed out medals  for their "heroic" service.