Pentagon Press Secretary John Kirby said Tuesday that the U.S. has seen "increased activity" and "offensive activity" by Russian forces in the Donbas region of Ukraine.

    The Donbas region, which has a large population of Russian-speaking citizens, has been at the center of tension in Ukraine for much of the conflict. Before launching its full-scale invasion of Ukraine, Russia moved to acknowledge claims of independence by separatists groups in the region.