The Americans, Andy Tai Ngoc Huynh and Alexander Drueke, are both from Alabama.
Russian President Vladimir Putin's "special military operation" into neighboring Ukraine began on Feb. 24, with Russian forces invading from Belarus, to the north, and Russia, to the east. Ukrainian troops have offered "stiff resistance," according to U.S. officials.
The Russian military has since launched a full-scale ground offensive in eastern Ukraine's disputed Donbas region, capturing the strategic port city of Mariupol and securing a coastal corridor to the Moscow-annexed Crimean Peninsula.
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A look at the two leaders at the center of the war in Ukraine and how they both rose to power, the difference in their leadership and what led to this moment in history.
Senior U.S. defense officials believe Russia's sunken Moskva cruiser was hit by two Ukrainian-launched Neptune missiles, bolstering claims from Ukrainian officials on Thursday.
The officials could not confirm how many Russian crew members were injured or killed in the attack or subsequent fire and eventual sinking.
-ABC News' Matt Seyler
The U.S. received a formal diplomatic note from Russia protesting U.S. and NATO military aid to Ukraine and accused them of violating principles and potentially risking weapons falling into bad actors' hands, a U.S. official briefed on the note confirmed to ABC News.
The Russian government previously warned the U.S. and NATO against providing weapons and other military equipment. Both Russian Foreign Minister Sergei Lavrov and one of his deputies, Sergei Ryabkov, also warned that Western convoys providing military aid were "legitimate targets" for Russian attack.
The White House referred questions to the State Department, but a State Department spokesperson declined to comment on "any private diplomatic correspondence."
State Department spokesperson Ned Price also declined to confirm the report, but during an interview with CNN, said, "The Russians have said some things privately. They have said some things publicly. Nothing will dissuade us from the strategy that we've embarked on."
Another source said it shows the effectiveness of U.S. weapon deliveries and security assistance, that Russia is upset is happening.
Price also told CNN that the war in Ukraine could drag on through the end of this year into next year, something other U.S. officials, including Chairman of the Joint Chiefs of Staff Mark Milley, have warned about as well.
"It is possible ... but what we're trying to do is to shorten this conflict," Price told CNN, by providing Ukraine with "unprecedented amount of security assistance" and applying pressure on Russia.
The Washington Post first reported on the diplomatic note.
-ABC News' Conor Finnegan, Matt Seyler, Justin Gomez and Shannon Crawford
Serhiy Volyna, commander of Ukraine's 36th Marine Brigade, which is engaged in a brutal fight to defend Mariupol against invading Russian forces, has issued an urgent plea for military reinforcements or a political solution -- anything to break Russia's siege of the Ukrainian port city.
In an interview Friday with Ukrainian online newspaper Ukrayinska Pravda, the commander said the situation is critical and the fighting is fierce.
"It can be done and it must be done as soon as possible," Volyna added.
Despite Russia's relentless bombardment for more than a month, the 36th Marine Brigade along with units of the Azov Regiment, a far-right group now part of the Ukrainian military, have held down Mariupol. They have refused to surrender, vowing to fight until the end.
-ABC News' Yulia Drozd
During his latest daily address, Ukrainian President Volodymyr Zelenskyy observed that Ukraine has withstood 50 days of the Russian invasion.
"During the 50 days of this war, Ukraine became a hero for the whole free world," he said.
Zelenskyy said he was grateful to those who have supported the country, though said that during the last 50 days he's started to view world leaders in a different light.
"I have seen politicians behaving as if they had no power, and I have seen non-politicians who did more in these 50 days than some statesmen who claimed leadership," he said.
Zelenskyy praised how Ukraine has defended itself, including those "who have shown that Russian ships can go ... to the bottom only" -- a nod to the sinking of the Russian Black Sea Fleet flagship vessel, Moskva.
Ukrainian government officials had claimed on Wednesday its armed forces fired missiles that damaged the vessel.
-ABC News' Fidel Pavlenko
