The United States on Thursday urged Russia to treat any American volunteers captured while fighting alongside Ukrainian troops as prisoners of war guaranteed humane treatment.
The State Department also said that a third American was believed to be missing in Ukraine in addition to two military veterans who were reportedly seized by Russian forces in a pitched battle last week.
"The Russians have certain obligations and members of the Ukrainian armed forces -- including volunteers who may be third-country nationals incorporated into the armed forces -- should be treated as prisoners of war under the Geneva Conventions," State Department spokesman Ned Price told reporters.
Prisoners of war must be "afforded the treatment and protections commensurate with that status, including humane treatment and fundamental process and fair-trial guarantees," he said.
Families and members of Congress said Wednesday that Alexander Drueke and Andy Huynh, both US military veterans who had been living in Alabama, lost contact with their relatives last week while fighting with Ukrainian forces near the Russian border.
Price said the United States could not confirm details on the pair and also that there were reports of a third US citizen who was said to have gone missing "in recent weeks."
The Geneva Conventions, originating in the 19th century and updated after World War II, define the rights of prisoners of war including barring torture and guaranteeing medical treatment.
Under former president George W Bush, the United States controversially labelled fighters detained in the "war on terror" as enemy combatants rather than prisoners of war, circumventing protections under the Geneva Conventions.
President Joe Biden's administration, while sending billions of dollars worth of weapons and economic aid to Ukraine, argues that the United States is not directly fighting Russia and has discouraged Americans from traveling to the war zone.
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