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

White House national security advisor Jake Sullivan hinted Thursday of more sanctions coming against Russia in the "next week or two" aimed at targeting ways Moscow is evading sanctions already imposed.

“Where our focus will be over the course of the coming days is on evasion,” Sullivan said Thursday at the Economic Club of Washington. “As Russia tries to adjust to the fact that it’s under this massive economic pressure, what steps do they take to try to evade our sanctions and how do we crack down on that? And I think we'll have some announcements in the next week or two that identify targets that are trying to facilitate that evasion both inside Russia and beyond."

When Sullivan was asked whether sanctions will automatically be lifted if a negotiated peace deal between Russia and Ukraine is worked out, he appeared cautious with his words, saying, “a lot of that depends on what the shape and scope” of the agreement is.

“A lot of it depends on what the Ukrainians, in consultation with us and the Europeans come to agree to," Sullivan said. "You know, we're not going to do a deal over the head of the Ukrainians where we give a bunch of sanctions relief to Russia. But if some measure of sanctions relief were built in to some credible diplomatic solution led by the Ukrainians, that's something that we would happily discuss."

But Sullivan said Russian oligarchs shouldn't expect to ever get back their yachts and other assets seized under sanctions that have been imposed, saying the ultimate goal is "not to give them back” once the war is over.

“The president is actively looking at how we can deal with the fact that as we seize these assets, our goal is not to give them back. Our goal is to put them to a better use than that," Sullivan said. "But I'll be careful in what I say today because there's an ongoing kind of policy process around how we end up dealing with that question. But, rest assured, that the goal is not just to sit on them for a while."

-ABC News' Justin Gomez