Two former soldiers of the United States, fighting alongside Ukrainian armed forces have been reportedly captured by the Russian forces.

Relatives of U.S. military veterans Andy Huynh, 27, and Alexander Drueke, 39, both from Alabama, said on June 15 that they last heard from the two men one week ago. They did not return from a mission around the Kharkiv region after fighting in an area that reportedly was hit hard by Russian strikes, the relatives said.

The White House said on June 16 that it’s “working very hard to learn more” about the two U.S. citizens, and the U.S. State Department is investigating reports that Russian or Russian-backed separatist forces captured at least two U.S. citizens, a spokesman said.

Russian Foreign Ministry spokeswoman Maria Zakharova said on June 16 in a televised program that Washington had not contacted Moscow regarding the two U.S. citizens. If Huynh and Drueke have been captured, they would be the first confirmed U.S. citizens to have been taken as prisoners in Russia’s ongoing invasion of Ukraine launched on February 24.

Last week, two British citizens, Aiden Aslin and Shaun Pinner were sentenced to death by forces of the DPR for “mercenary activities.