Russia-Ukraine updates: Russian missiles hit close to nuclear reactors: IAEA director

Senior State Department officials said Tuesday that the U.S. has not received any direct information from Russian authorities or any of their proxy forces about Alexander Drueke and Andy Huynh, two American citizens feared to have been captured while fighting in Ukraine.

While the Kremlin's spokesman has publicly labeled Drueke and Huynh as mercenaries and said "they should be held responsible," Moscow has not yet officially confirmed to the U.S. that the men have been captured.

But senior State Department officials reiterated that the U.S. has been in contact with the Russian government to remind them of the protections granted to prisoners of war under the Geneva Conventions, even though the Kremlin argues that as "soldiers of fortune" the detainees would not be covered by those protections and could face the death penalty.

"We obviously disagree vigorously. And we have made our position clear to the Russian government," one official said.

The Geneva Conventions outline the humanitarian rights given to prisoners of war, however, mercenaries are not given the same protections.

When asked if the Americans could face the death penalty, Kremlin spokesman Dimitry Peskov said this week, "Yes, we do not rule out anything."

White House spokesman John Kirby on Tuesday called that "appalling."

"It's appalling that a public official in Russia would even suggest the death penalty for two American citizens that were in Ukraine," he said.

Kirby added that the U.S. was still "trying to learn more about these two individuals."

-ABC News' Shannon Crawford and Ben Gittleson

Russia now controls about 96% of the Luhansk region in eastern Ukraine.

Ukrainian forces are fighting to hold onto several municipalities in the Luhansk region: Severodonetsk, Lysychansk and the three neighboring villages of Pidlisne, Myrna Dolyna and Toshkivka.

Serhiy Haidi, the head of the Luhansk Regional State Administration, said Ukrainian forces are still holding on to the city of Lysychansk because it sits on elevated ground, giving them a tactical advantage. Russian forces have been heavily shelling the city where approximately 10,000 civilians remain. More than 10 high-rise buildings, private homes and a police station have been destroyed.

In Severodonetsk, Russian forces continue to fire on the Azot chemical plant where 568 civilians are sheltering.

-ABC News' Fidel Pavlenko, Natalya Kushnir and Christine Theodorou

Attorney General Merrick Garland made an unannounced visit to Ukraine on Tuesday.

Garland is meeting with Ukrainian Prosecutor General Iryna Venediktova to discuss the efforts to help identify, apprehend and prosecute the people involved in war crimes and other atrocities in Ukraine, a Department of Justice official said.

Garland told reporters, "The United States is sending an unmistakable message: There is no place to hide. We and our partners will pursue every avenue available to ensure that those who are responsible for these atrocities are held accountable."

Garland announced that the Justice Department has launched a War Crimes Accountability Team.

"This initiative will bring together the Department’s leading experts in investigations involving human rights abuses and war crimes and other atrocities; and provide wide-ranging technical assistance, including operational assistance and advice regarding criminal prosecutions, evidence collection, forensics, and relevant legal analysis," the department said in a statement. "The team will also play an integral role in the Department’s ongoing investigation of potential war crimes over which the U.S. possesses jurisdiction, such as the killing and wounding of U.S. journalists covering the unprovoked Russian aggression in Ukraine."

The Justice Department also said it will provide more personnel to help Ukraine and other international partners counter Russian illicit finance and sanctions evasion, including "an expert Justice Department prosecutor to advise on fighting kleptocracy, corruption, and money laundering."

-ABC News' Alex Mallin

U.S. citizen Stephen Zabielski has died in Ukraine, the State Department confirmed to ABC News Tuesday morning.

"We can confirm the death of U.S. citizen Stephen Zabielski in Ukraine," a State Department spokesperson said. "We have been in touch with the family and have provided all possible consular assistance. Out of respect to the family during this difficult time, we have nothing further."

Zabielski's death was first reported by Rolling Stone.

US officials again cautioned Americans against traveling to Ukraine, saying "that U.S. citizens in Ukraine should depart immediately if it is safe to do so using any commercial or other privately available ground transportation options."

-ABC News' Matthew Seyler