Ukraine's military took a defiant stand this weekend -- refusing to give in to Russia's demand for Kyiv's troops in the port city of Mariupol to surrender — at the same time that President Joe Biden and his allies face a new precipice in deciding how far the US can go in arming the embattled country, as Russia signals that it may take more aggressive action to stop the flow of weapons from the US and NATO.

There are new worries about how quickly Ukraine could run out of ammunition as heavier fighting intensifies in the Donbas where Russia is trying to encircle and cut off Ukrainian forces in their quest to control that region.

As he tries to keep the pressure on allies to lend greater support in this next phase, Ukrainian President Volodymyr Zelensky is arguing that the West must view that fight as a critical pivot point in curbing the unbridled ambitions of Russian President Vladimir Putin and demonstrating the West's commitment to defending democracy against a voracious autocratic power.

Zelensky warned that the battle ahead in the Donbas “can influence the course of the whole war” and said his country has no intention of giving up territory in the eastern part of Ukraine to end the war during an exclusive interview with CNN's Jake Tapper that aired Sunday on CNN's “State of the Union.”

If Russia is able to capture the Donbas region, he warned, it is entirely possible that Putin could renew his attempt to take control of Kyiv. When pressed by Tapper on whether he was satisfied with the US announcement last week of another $800 million in military aid to bolster Ukraine's forces in the Donbas, Zelensky replied, “of course we need more.”

“There will never be enough. Enough isn't possible,” Zelensky said, as he explained the challenges that lie ahead in the eastern region of his country. “There is a full-scale war ongoing today, so we still need a lot more than what we have today. ... We do not have technical advantages over our enemy. We're just not on the same level there.”

“For Biden's confirmed $800 million in support, what's most important is speed,” he added.

But even as that latest aid has begun arriving in the region, CNN's Barbara Starr reported this weekend that there is rising concern about how quickly Ukraine could deplete its stores of ammunition in this next battle.

Though the US announced that it was sending 18 155mm Howitzer cannons and 40,000 artillery rounds as part of its latest package, Starr reported that a US official warned that the aid could be used up within a matter of days as heavy fighting intensifies in the Donbas.

Given those pressures, US officials must be clearer in defining their objectives and whether America is committed to doing what it takes to help Ukraine win, retired Lt. Gen. Ben Hodges, the former commanding general of the US Army in Europe, said Sunday in an interview on CBS' “Face the Nation with Margaret Brennan.” While the latest round of US aid was “substantial,” he said it was not enough.

“What the Ukrainians need desperately are long-range fires, rockets, artillery, drones that can disrupt or destroy the systems that are causing so much damage in Ukrainian cities, and which will also play a critical role in this next phase, if and when it begins,” Hodges said. “I would really like to hear the administration talk about winning and having a sense of urgency on getting these things there. Otherwise, this window of opportunity we have, the next couple of weeks, to really disrupt Russia's attempt to build up is going to pass.”

A 'red line' in Mariupol

A critical piece of Russia's current campaign is capturing the port city of Mariupol in an effort to create Putin's desired land bridge from eastern Ukraine all the way to the Crimean peninsula. Russia's Ministry of Defense had demanded that Ukrainian soldiers in Mariupol surrender by 1 p.m. local time on Sunday, but then said in a statement that the ultimatum was ignored.

In its statement, Russia's Ministry of Defense said it had surrounded the remaining Ukrainian soldiers and others who have been holding out at the Azovstal steel plant. “In case of further resistance, all of them will be eliminated,” the statement said.

An adviser to Mariupol's mayor said Sunday that Russian forces have announced that the city will be closed for entry and exit on Monday and that they had begun issuing passes that would be required to move within the city itself.

Both Zelensky and Ukrainian Foreign Minister Dmytro Kuleba framed the fate of Mariupol as another critical turning point in the war -- in part because the human toll of Russia's relentless shelling of that city is still unknown.

Zelensky has previously warned that the elimination of military forces in Mariupol could bring any further peace negotiations with Russia to a halt. On Sunday, Kuleba noted that it was hard for his country to continue talks with Russia after the atrocities in Bucha. Russia's determination to raze Mariupol “to the ground at any cost” could become “a red line,” he said during an interview on CBS' “Face the Nation with Margaret Brennan.”

In a chilling admission, Zelensky told Tapper that no one yet knows how many people have died in Mariupol. “If anyone gives you a figure, it would be a total lie,” Zelensky said. He added that “several thousand, tens of thousands” were forced to evacuate the city in the direction of Russia, leaving no document trail, and that the Ukrainian government does not know where they are.

“About 5,000 children deported from this region to Russia's side because they didn't allow them to go to the Ukraine side,” Zelensky said in the interview. “(Those) children. Where are they? Nobody knows.”

While he said he was still prepared to engage in diplomatic discussions with Russia if that opportunity arises, it has become harder to do so as he has watched the staggering toll of Putin's aggression on his country. “What's the price of all this? It's people. The many people who have been killed,” Zelensky said. “And who ends up paying for all of this? It's Ukraine. Just us.”

Putin's hardened mindset

One of the greatest challenges for the Biden administration and its allies thus far has been determining where Putin's "red line" lies and how much they can continue to assist Ukraine without provoking the Russian president to widen the war, potentially placing NATO troops in harm's way.

As the US prepared to send the $800 million aid package last week, Russia warned in a diplomatic note to the State Department that there would be "unpredictable consequences" if the US and allies continue to send in the heavier duty weaponry that Ukraine has sought.

Military experts interpreted the demarche as sign that Russia could contemplate targeting not only the weapons themselves as they arrive on Ukrainian soil, but also NATO supply convoys that ferry the weapons to Ukraine's borders.

As world leaders try to glean what Putin is thinking -- and how far he might go in trying to punish the nations that help Ukraine -- Austrian Chancellor Karl Nehammer, who met face-to-face with Putin last week, said it was clear that Putin believes he's winning the war and is operating “in his own war logic.”

“He thinks the war is necessary for security guarantees for the Russian Federation. He doesn't trust the international community. He blames the Ukrainians for genocide in the Donbas region,” Nehammer said on NBC's “Meet the Press” Sunday, referring to the fictional propaganda that Putin has spouted to justify his acts of aggression against Ukraine. “He is now in his world, but I think he knows what is going on now in Ukraine.”

Given the immense challenges of facing off against a leader with that warped and rigid mindset, Zelensky is trying to persuade world leaders to become more involved in the next phase by warning that they should be worried about the possible consequences of Putin's next steps -- including that he could use a tactical nuclear weapon because he has shown so little regard for human lives during his invasion of Ukraine.

Zelensky also issued a challenge to Ukraine's allies when Tapper asked him whether the promise that world leaders make each year on Holocaust Remembrance Day -- in the refrain "Never Again" -- now rings hollow given that their efforts so far have failed to stop the atrocities that Russia has inflicted throughout the course of its unprovoked invasion.

“I don't believe the world,” Zelensky said plainly when asked about that refrain. “Never again. Really, everybody is talking about this and yet, as you can see, not everyone has got the guts.” (CNN)

PHOTO CAPTION: Zelensky rejects calls to give up Donbas area to end war (CNN)Ukraine's military took a defiant stand this weekend -- refusing to give in to Russia's demand for Kyiv's troops in the port city of Mariupol to surrender — at the same time that President Joe Biden and his allies face...

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