Two U.S. citizens who traveled to Ukraine as volunteer fighters against Russian forces have been missing for a week and are feared captured, family members said on Wednesday.

Alexander Drueke, 39, of Tuscaloosa, Alabama; and Andy Huynh, 27, of Hartselle, Alabama, were last in contact with their families on June 8 and did not return from a mission around the Kharkiv region of eastern Ukraine.

Reports that the two men have been taken prisoners of war by Russia are unconfirmed, the families and a U.S. State Department spokesperson said.