The White House on Wednesday urged Americans not to travel to Ukraine after reports emerged that two Americans had been captured by Russian forces.
John Kirby, a National Security Council spokesperson, told reporters that if the reports are true, the United States "will do everything we can" to get them back.
The men were identified by CNN as Alexander John-Robert Drueke, age 39, from Tuscaloosa, Alabama, and Andy Tai Ngoc Huynh, age 27, from Hartselle, Alabama. The two were fighting with Ukrainian troops north of Kharkiv in eastern Ukraine under the command of the 92nd mechanized brigade but have been missing for nearly a week.
"What we know officially at this point from the State Department is that Andy and Alex are missing," Joy Black, Andy's fiancee, said by phone.
"We do not have confirmation for anything beyond that. Obviously the longer the search goes the more we start to consider other scenarios," she added.
Russia's Ministry of Foreign Affairs spokeswoman Maria Zakharova said the United States has not contacted Russia regarding the reports of the US fighters.
"I don't have that information, I check every day, and I'll check today. We make all information about the fate of detained mercenaries or those sentenced to trial public," the RIA news agency reported Zakharova as saying.
Russia's defense ministry did not immediately respond to a request for comment.
Their passports and entry stamps into Ukraine were provided to CNN by their acting sergeant, who further said that they had gone missing during clashes with Russian troops on June 9 near the town of Izbytske.
If the pair have been captured, they would be the first confirmed US citizens to have been taken as prisoners of war in the conflict that began on February 24 when Russian President Vladimir Putin ordered an invasion of its neighbor.
This is a developing story.