Ukraine's Zelenskyy warns Putin will push Russia's war "very quickly" onto NATO soil if he's not stopped

Ukraine's President Volodymyr Zelenskyy met our CBS News team at an undisclosed, bombed-out building in the far east of his country. Bombed-out buildings aren't hard to come by here.

With spring approaching, Zelenskyy said Ukraine's forces had managed to hold off Russian advances through the worst of the winter months.

"We have stabilized the situation. It is better than it used to be two or three months ago when we had a big deficit of artillery ammunition, different kinds of weapons," he said, "We totally didn't see the big, huge counteroffensive from Russia... They didn't have success."

"We need help now": Zelenskyy says Russian offensive looming
But Zelenskyy acknowledged that the invading Russian troops and their seemingly endless supply of missiles and shells had destroyed "some villages."

"We didn't have rounds, artillery rounds, a lot of different things," he said, stressing that while his troops have managed to keep the Russians largely at bay up to now, they're not prepared to defend against another major Russian offensive expected in the coming months.

That, he said, was expected around the end of May or in June he said.

"And before that, we not only need to prepare, we not only need to stabilize the situation, because the partners are sometimes really happy that we have stabilized the situation," Zelenskyy said of the U.S. and Ukraine's other backers. "No, I say we need help now."

In what has become a grinding artillery war of attrition, Russia not only has the upper hand with more firepower, but also firepower with a longer reach.

"In Bakhmut and Avdivka and Lysychansk and Soledar and so on, it was really hard to fight the adversary, whose artillery shell can fire 20-plus kilometers, and [our] artillery shell is 20-minus," he said.

With heavily armed soldiers keeping watch on the horizon, we joined Zelenskyy as he inspected freshly dug underground bunkers in Ukraine's northeast, on the outskirts of the city of Sumy, no more than 15 miles from the Russian border.

The entire area is on a war footing in response to a significant buildup of Russian troops just across that border, and attacks on nearby villages, Zelenskyy told us.

"Usually, when they attack by artillery and destroy the villages, after that, they always tried to occupy," he said. "We don't know what will be tomorrow. That's why we have to prepare."

Zelenskyy on the stalled U.S. aid, and why Ukraine needs it
He said what's needed most are American Patriot missile defense systems, and more artillery. While he's grateful for the billions of dollars in U.S. support his country has already received, he said the nature of the funding dedicated by the American government to help Ukraine must be put into perspective.

"Dozens of billions remain in the U.S.," he said. "Let's be honest, the money which is allocated by the Congress, by the administration, in the majority of cases, 80% of this money — well, at least more than 75% — stays in the U.S. This ammunition is coming to us, but the production is taking place there, and the money stays in the U.S., and the taxes are staying in the U.S."

"Yes, it's a huge support coming to us, but we need [it]," added the president.

With lawmakers in the U.S. still wrangling after months of partisan gridlock over a $60 billion aid package, Zelenskyy acknowledged that the war in Gaza had refocused global attention — and U.S. aid — away from his country's struggle.