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

In a televised announcement on Tuesday, Anatoliy Fedoruk, the mayor of Bucha, Ukraine, said that the bodies of 403 people presumably killed by Russian forces have been recovered in his city and that he expects the number to rise.

Fedoruk said 16 residents of Bucha remain unaccounted for and are presumed dead.

He said 163 of the 403 bodies recovered have been identified.

Fedoruk alleged last week that nearly all of those killed in Bucha are civilians.

Surviving residents of the besieged town told an ABC News crew in Bucha last week that Russian troops allegedly tortured people before killing them and executed many men under the age of 50.

When ABC News arrived in Bucha on Tuesday, bodies still lay in the streets.

Russia has denied committing atrocities in Ukraine and said it is not targeting civilians.

-ABC News' Christine Theodorou

A senior U.S. defense official said Tuesday that the United States cannot confirm whether Russia used chemical agents in Mariupol or elsewhere in Ukraine but that the Pentagon is concerned Russian forces could disguise such attacks.

The Department of Defense has seen evidence Russia has considered disguising the use of chemical weapons by making them appear to be more benign riot control agents, the official said.

"In the past we've had indications that that could be one thing that the Russians look at is the potential mixing of agents with the idea that they could disguise a more serious attack by using the vehicle and the techniques of riot control agents," the official said.

Pentagon press secretary John Kirby released a similar statement Monday night to address social media reports claiming Russia used a chemical weapon in Mariupol.

"These reports, if true, are deeply concerning and reflective of concerns that we have had about Russia’s potential to use a variety of riot control agents, including tear gas mixed with chemical agents, in Ukraine," Kirby said.

-ABC News' Matt Seyler

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