Russia-Ukraine updates: 2 US veterans who joined Ukrainian forces missing

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

Viktor Medvedchuk, a pro-Russian oligarch and personal friend of Vladimir Putin, has been captured in Ukraine, Ukrainian President Volodymyr Zelenskyy said.

Zelenskyy posted a photo on his official Telegram account of the captured Medvedchuk in handcuffs and wearing military fatigues.

The photo was accompanied by a caption praising the Security Service of Ukraine's "special operation" that led to Medvedchuk's capture. "Well done! Details later. Glory to Ukraine," the caption reads.

Medvedchuk is a pro-Russian Ukrainian politician and leader of Ukraine's Opposition Platform.

In May 2021, Ukraine indicted Medvedchuk on charges of treason and attempting to steal natural resources from Russia-annexed Crimea. He was initially placed under house arrest in Ukraine but escaped just days after the Russian invasion began on Feb. 24.

Medvedchuk's detention was also confirmed by Ukraine's Security Service on their official Facebook page. The agency said Medvedchuk was wearing a uniform from the Ukrainian armed forces to disguise himself.

Medvedchuk is a business oligarch in Ukraine with very close ties to Putin. The Ukrainian National News Agency reported that Putin is the godfather of one of Medvedchuk's daughters.

President Joe Biden spoke with British Prime Minister Boris Johnson by phone Tuesday about the need to accelerate military and economic assistance to Ukraine, according to a spokesperson for Johnson.

"The Prime Minister updated President Biden on his recent visit to Kyiv, and said he had been humbled by President Zelenskyy’s strength and resolve," the spokesperson said.

The leaders discussed the need to accelerate assistance to Ukraine, including bolstering military and economic support as Ukrainian forces prepare for another Russian onslaught in the east of the country.

"The prime minister paid tribute to the U.S.’ colossal military contribution to Ukraine, and updated on the U.K.’s new package of support, including anti-ship missiles and military vehicles, which would arrive in the coming days and weeks," the spokesperson said. "Both leaders were clear that Putin would never be able to hold down the spirit of the Ukrainian people, despite his monstrous attempts."

The most recent $800 million U.S. military aid package for Ukraine has mostly been delivered and will be completed in the coming days, a U.S. defense official said Tuesday.

"Yesterday, two U.S. flights arrived in the region with everything from small-arms ammunition, machine guns, body armor, grenades and other explosives," the official said.

So far, the United States has sent 19 out of an expected 20 flights needed to deliver the $800 million package.

Johnson, according to his spokesperson, told Biden that a long-term commitment to Ukraine was needed from the international community to ensure the Ukrainian people’s vision for their country’s freedom can be realized.

“The pair also agreed to continue joint efforts to ratchet up the economic pressure on Putin and decisively end Western reliance on Russian oil and gas," Johnson's spokesperson said.

-ABC News' Matt Seyler