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."

The United States announced an additional $2 billion military aid package for Ukraine on Friday, as the Eastern European country marks the one-year anniversary of Russia's full-scale invasion.

The new aid package includes more missiles for Ukraine's U.S.-supplied High Mobility Artillery Rocket Systems (HIMARS), additional 155mm artillery ammunition, more Switchblade one-way drones and other military equipment, according to the U.S. Department of Defense.

"One year into a war of aggression waged by a permanent member of the U.N. Security Council, our allies and partners worldwide stand united and resolute," U.S. Secretary of Defense Lloyd Austin said in a statement. "Putin's reckless, illegal war is not just an all-out assault on Ukraine's sovereignty and a historic threat to European security. It is also a direct attack on the system of rules, institutions, and laws that the world built at such great cost after World War II -- a system that rejects aggression and respects the rights of all countries, big and small."

The additional aid is being provided under the Ukraine Security Assistance Initiative (USAI) plan, meaning this equipment and artillery have to be made from scratch before being delivered, which will take time. This is different from the other aid packages that come from existing U.S. military stockpiles and get delivered faster.

With the new aid package, the Biden administration has now provided $31.8 billion in assistance to Ukraine since Russia's war began on Feb. 24, 2022.

"Putin thought that Ukraine’s defenses would collapse, that America's resolve would falter, and that the world would look the other way. He was wrong," Lloyd said. "One year later, Ukraine's brave defenders have not wavered, and neither has our commitment to support them for as long as it takes. Despite the Kremlin's campaign of cruelty, the people of Ukraine have shown stunning bravery, skill, and fortitude. Today and every day, we stand by the courageous Ukrainians fighting to defend their country, and we mourn with those who have lost their loved ones in Moscow's monstrous and unnecessary war."

"Difficult times may lie ahead, but let us remain clear-eyed about what is at stake in Ukraine," he added. "And let us remain united in purpose and in action -- and steadfast in our commitment to ensure that a world of rules and rights is not replaced by one of tyranny and turmoil."

-ABC News' Luis Martinez