Two men were killed and at least 40 people, including children, were injured after Russian missiles struck Pavlograd, Ukrainian President Volodymyr Zelenskyy and other officials said.

Serhii Lysak, the head of the military administration of the Dnipropetrovsk, said 19 high-rise buildings, 25 private houses, six schools and preschool education institutions and five shops were hit by the missiles.

Five children were among the wounded officials said. The youngest victim is 8 years old, according to officials.

-ABC News' Wil Gretsky

The U.S. estimates that Russia has suffered over 100,000 casualties, including over 20,000 killed in action, since it stepped up its winter offensive in December, a White House National Security Council told ABC News.

Half of the 20,000 killed in action were members of the Russian-backed private military Wagner Group, according to White House spokesman John Kirby. The majority of Wagner fighters killed were allegedly ex-convicts, according to Kirby.

Kirby said that the data came from "some downgraded intelligence" that the U.S. has been able to collect. He was unable to provide data on deaths of Ukrainian fighters.

Kirby had earlier told reporters that the Russian casualties took place from battles in Bakhmut since December, however, the National Security Council member said the data encompasses all winter battles.

Many, but not all, of these Russians were injured or killed in the Bakhmut fighting, according to the official.

Kirby emphasized that the U.S. thinks Bakhmut holds "very little strategic value for Russia" and if captured by Russia, it "would absolutely not alter the course of the war in Russia’s favor."

-ABC News' Ben Gittleson

A Russian missile airstrike was reported in different areas of Ukraine Sunday evening.

Sixteen Russian Tu-95 bombers were reported in the air from various airfields and explosions were reported in the Dnipropetrovsk region of Ukraine at 10:51 p.m. local time.

“Presumably, Kh-101 cruise missiles are actively flying at the Pavlograd-1 and Pavlograd-2 railway stations, where trains with APU (Ukrainian Armed Forces) equipment and people were located,” a Russian Telegram channel, Military Chronicle, said.

Several explosions were heard in the city of Pavlograd, in the Dnipropetrovsk region, local Ukrainian media reported.

An air alert was announced for the region at 9:30 p.m. local time and about 10:00 p.m., social networks began to report explosions in Pavlograd.

According to local media, repeated explosions were heard in the city at 10:20 p.m.

The strikes destroyed Ukrainian anti-aircraft missiles for the S-300 complex on Pavlograd, Russian Telegram channel Intel Slava reported.

S-300s are long-range surface-to-air missiles.

Supply vehicles with reserve ammunition that belong to the Ukrainian Armed Forces were also hit, the channel said, citing eyewitness accounts.

Intel Slava is funded by the Russian government.

“The detonation of rockets has been going on for almost an hour,” the Intel Slava post said.

It's unclear how many people were injured or killed.

-ABC News' Fidel Pavlenko and Anastasia Bagaeva

The Russian oligarch behind the Wagner private paramilitary group fighting for the Kremlin in Ukraine is threatening a mutiny if his forces are not resupplied with ammunition soon.

Yevgeny Prigozhin, owner and curator of the Wagner group, penned a letter to Russian Defense Minister Sergei Shoigue, demanding ammunition be provided to his forces on the battlefield.

In the letter, Prigozhin wrote that if supply problems are not fixed fast, he will complain to Russian President Vladimir Putin and his mercenaries would abandon their positions in Bakhmut, Ukraine, where heavy fighting has been going on for weeks, The Moscow Times reported.

“I appeal to Shoigu with a request to immediately issue ammunition. In case of refusal, I consider it necessary to convey to the Supreme Commander-in-Chief information about the existing problem in order to make a decision and about the advisability of further presence of Wagner PMC in Bakhmut in the conditions of a shortage of ammunition," Prigozhin wrote.

He added, "If the deficit is not replenished ... we will be forced to withdraw part of the units from this territory, and then everything else will crumble. Therefore, the bell is already ringing -- it is called an alarm."

Emphasizing the urgency, Prigozhin noted that Ukraine is planning to launch a counteroffensive soon.

There was no immediate public response from Shoigu or the Kremlin.

"We need to stop deceiving the population and telling that everything is fine with us," Prigozhin wrote. "I must honestly say: Russia is on the brink of disaster. If these screws are not adjusted today, the 'aircraft' will crumble in the air."

One year ago, with Ukraine's borders surrounded by what seemed to be a superior military force, many U.S. officials and analysts predicted a swift Russian blitz to Kyiv.

But after Russian President Vladimir Putin sent his more than 150,000 arrayed troops across the border, it soon became clear that a dual reassessment was in order: The Russian invaders were less potent than advertised, and the Ukrainians were unexpectedly stubborn and wily in the defense.

Some of the Russian troops weren’t even aware they were on a combat mission until Ukrainian bullets came cracking past them, according to U.S. officials. Meanwhile, Ukrainian forces stalled a massive Russian supply convoy through direct attacks and by destroying a key bridge. Only one week into the invasion, Putin's men were plagued with food and fuel shortages, morale running similarly low.

Kyiv stood.

"Putin assumed that Ukraine was an easy target, Putin assumed that Kyiv would easily fall, and Putin assumed that the world would stand by," U.S. Secretary of Defense Lloyd Austin said during a speech in Brussels last week. "But the Kremlin was wrong on every count."

Ukrainian forces were armed with more than grit.

They also had years of U.S. and NATO military training, plus American-made weapons, like anti-armor Javelins and anti-aircraft Stinger missiles. These made Russian vehicles vulnerable to ambush, and left Russian helicopter and jet pilots wary of flying over Ukrainian positions. Indeed, many airmen did not return from their sorties.

Despite astonishing losses of soldiers and vehicles, Putin has shown no inclination to end the conflict anytime soon. And despite its tenacity, Ukraine has also taken significant casualties, and is not able to produce enough of its own weapons and ammunition to keep up the fight.

Ukraine, after thwarting the advance on its capital, and later routing Russian forces from Kharkiv, now largely faces a battle of supply.

"When this war began, Russia had a larger population, a much bigger defense budget, a bigger military, bigger industrial base. So, this became an industrial war and a war of industrial bases," said Seth Jones, director of the International Security Program at the Center for Strategic and International Studies. "This is why Western industrial support has been so critical."

A key question now is, despite massive military aid packages and a promise to send even more, could the U.S. strategy ultimately result, not in a Ukrainian victory, but a stalemate in a years-long war of attrition?

-ABC News' Matt Seyler