Washington and its European allies have provided weapons and financial assistance to Ukraine but avoided direct involvement in the conflict. Some American citizens, however, have volunteered to fight for Ukraine.
On Monday, the Kremlin said two Americans detained in Ukraine were mercenaries not covered by the Geneva convention who should face responsibility for their actions.
Kremlin spokesman Dmitry Peskov's comments were the first formal acknowledgment that the two, identified in U.S. reports as Andy Huynh, 27, of Hartselle, Alabama, and Alexander Drueke, 39, of Tuscaloosa, Alabama, were being held.
A U.S. State Department spokesperson said they had been in touch with Russian authorities regarding any U.S. citizens who may have been captured.