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

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

U.S. Secretary of State Antony Blinken was meeting Tuesday morning with U.N. refugee agency chief Filippo Grandi amid the crisis in Ukraine and other upheavals that have displaced people around the globe.

"We've only seen that challenge increase and, of course, Ukraine is now added to the mix with Russian aggression displacing, within Ukraine or outside of Ukraine, two-thirds of the children in that country, as well as, of course, many, many adults," Blinken said while sitting across the table from Grandi.

There are some 95 million people displaced across the globe, with the number of refugees alone larger than the populations of Spain or South Korea, Blinken said.

Blinken added the United States is "grateful" for the work the U.N. High Commissioner for Refugees is doing to meet the needs of refugees. He said the United States is working with the agency to both resettle refugees in the United States and care for refugee populations overseas.

Grandi praised the United States for being the largest donor and the largest resettlement country for refugees.

But weeks after the Biden administration said it would admit up to 100,000 Ukrainian refugees, Grandi said the White House has released no details about how the United States will do that.

"This figure that he (Blinken) mentioned -- 95 million -- maybe 96 million by today, who knows?" Grandi said, adding that the number of refugees had gone up by 12 million in less than two months with the crisis in Ukraine.

Grandi noted other crises from Afghanistan to Africa and Venezuela that have displaced people and said of Russia's war in Ukraine, "That crisis should not make us forget everything else."

-ABC News Conor Finnegan