Russia claimed Wednesday that more than a thousand Ukrainian soldiers had surrendered in Mariupol, a besieged port city in southeastern Ukraine that is still held by Ukrainian forces.

"In Mariupol city, near the 'Illich' Steelworks, 1,026 Ukrainian servicemen of the 36th Marine Brigade have voluntarily laid down their arms and surrendered as a result of a successful offensive by the Russian Armed Forces and Donetsk People's Republic militia units," the Russian Ministry of Defense said in a statement.

The surrendering troops included 162 officers and 47 women, according to the ministry.

"151 wounded Ukrainian servicemen of the 36th Marine Brigade received primary medical care immediately on the spot, after that they were all taken to the Mariupol city hospital for further treatment," the ministry added.

The Biden administration could announce as early as Wednesday upward of $750 million in additional military assistance to Ukraine, a U.S. official told ABC News.

The new assistance could possibly include a range of military hardware -- including howitzers, artillery and Humvees -- though the full package still needs to be finalized, the official said.

-ABC News' Luis Martinez

Correction: Mi-17 helicopters are not being considered as part of the package, as previously reported.

President Joe Biden used the word "genocide" for the first time to describe Russia's actions in Ukraine during remarks on Tuesday.

"Your family budget, your ability to fill up your tank -- none of it should on hinge on whether a dictator declares war and commits genocide half a world away," Biden said in Menlo, Iowa, during remarks primarily about the U.S. economy.

Biden was asked by reporters on April 4 if he thought the atrocities in Bucha were a genocide, as Ukrainian President Volodymyr Zelenskyy had claimed. "No, I think it is a war crime," Biden responded then.

That same day, Biden's national security adviser, Jake Sullivan, said the administration had not yet seen the "systematic deprivation of life" necessary to meet the definition of genocide.

Biden confirmed his word choice to the White House pool Tuesday evening before boarding Air Force One, saying that since last week the "evidence is mounting."

"Yes, I called it genocide," Biden said. "Because it has become clearer and clearer that Putin is just trying to wipe out the idea of even being able to be a Ukrainian."

He then qualified that the determination of genocide is officially up to legal experts, but that "it sure seems that way to me."

-ABC News' Sarah Kolinovsky

The U.S. State Department is condemning the arrest in Russia of Vladimir Kara-Murza, a Russian opposition activist and critic of the Kremlin's invasion of Ukraine.

State Department spokesperson Ned Price said Kara-Murza's arrest on Monday is another example of a Russian government "that is more aggressive beyond its borders and more oppressive within its borders."

He condemned Kara-Murza's arrest, noting that the activist has previously been arrested by Russian authorities and that he has survived two poisoning incidents.

"The Russian people -- and this is the key point -- like people everywhere, have the right to speak freely, to form peaceful associations, to exercise their freedom of expression and to have their voices heard through free and fair elections," Price said.

Kara-Murza, a Washington Post columnist who has testified before Congress, survived poisoning incidents in 2015 and in 2017. At the time of his second poisoning, Kara-Murza's wife, Evgenia Kara-Murza, gave an exclusive interview to ABC News in which she pleaded for then-President Donald Trump to support her husband and warned that Putin "cannot be dealt with on friendly terms."

Following her husband's arrest this week, Evgenia Kara-Murza posted a message on Twitter calling attention to her husband's arrest.

"Twice have the Russian authorities tried to kill my husband for advocating for sanctions against thieves and murderers, and now they want to throw him in prison for calling their bloody war a WAR. I demand my husband's immediate release!" Evgenia Kara-Murza, who lives in the Washington, D.C., area, tweeted.

-ABC News' Conor Finnegan

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan