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

As many as 1,700 Ukrainian soldiers have likely surrendered from the Azovstal Iron and Steel Works plant in war-ravaged Mariupol this week, according to the U.K. Ministry of Defense.

"An unknown number of Ukrainian forces remain inside the factory," the ministry said Friday in an intelligence update. "Once Russia has secured Mariupol, it is likely they will move their forces to reinforce operations in the Donbas."

For weeks, Ukrainian fighters and civilians were holed up inside the sprawling industrial site as the remaining pocket of resistance to Russia's relentless bombardment of Mariupol, a southeastern Ukrainian port city strategically located on the Sea of Azov between eastern Ukraine's disputed Donbas region and the Moscow-annexed Crimean Peninsula. Russia claimed Thursday that 1,730 Ukrainian soldiers had surrendered in Mariupol over the previous three days, while Ukraine confirmed Tuesday that more than 250 had yielded in the initial hours after it ordered them to do so.

Mariupol is the largest city that Russian forces have seized since launching an invasion of neighboring Ukraine on Feb. 24. Its complete capture gives Russia total control of the coast of the Sea of Azov as well as a continuous stretch of territory along eastern and southern Ukraine.

"Staunch Ukrainian resistance in Mariupol since the start of the war means Russian forces in the area must be re-equipped and refurbished before they can be redeployed effectively," the U.K. defense ministry said. "This can be a lengthy process when done thoroughly."

"Russian commanders, however, are under pressure to demonstrably achieve operational objectives," the ministry added. "This means that Russia will probably redistribute their forces swiftly without adequate preparation, which risks further force attrition."

Nearly 28,000 Ukrainian citizens have arrived in Belarus since Russia invaded neighboring Ukraine on Feb. 24, according to the Belarusian State Border Committee.

"Between 6 a.m. on February 24 and 6 a.m. on May 20, a total of 27,868 Ukrainian citizens arrived in Belarus, including 15,793 who crossed the Ukrainian-Belarusian border, 10,563 by transit through Poland, 1,305 through Lithuania, and 207 through Latvia," the committee said in a statement Friday.

In the past 24 hours alone, 154 Ukrainian citizens arrived in Belarus, including 120 via Poland, according to the committee.

Belarus shares a land border with both Ukraine and Russia, and is Moscow's main ally.

President Joe Biden will sign the $40 billion Ukraine aid bill while he's in Asia, a White House official said.

"The president does intend to sign the bill while he's on the road so that he can sign it expeditiously," national security adviser Jake Sullivan told reporters aboard Air Force One while en route to the region Thursday evening. "The modalities of that are being worked right now so that he can get it and sign it."

The bill, which passed the Senate earlier Thursday with bipartisan support, will need to be flown to the region so that Biden can sign it. The practice of flying bills to presidents for signature dates back to the Truman administration, but this is a first for Biden.

Biden departed for South Korea Thursday and will visit Japan later in the week during his first trip to Asia as president.

-ABC News' Sarah Kolinovsky

U.S. Secretary of State Antony Blinken announced Thursday that he has authorized a 10th drawdown of additional arms and equipment for Ukraine worth $100 million from U.S. Department of Defense inventories.

This brings total U.S. military assistance to Ukraine to approximately $3.9 billion in arms and equipment since Russia invaded Ukraine on Feb. 24.

"The United States is committed to helping Ukraine continue to meet its defense needs and build its future capabilities, as well as to bolster Allies and partners across NATO’s Eastern Flank and the broader region," Blinken said in a statement.

-ABC News' Shannon Crawford