Ukrainian tanks seen heading to Kramatorsk

STORY: After failing to seize Ukraine's capital Kyiv or its second city Kharkiv, Russia has poured thousands of troops into the region trying to take full control of the Donbas, comprised of two eastern provinces Moscow claims on behalf of separatists.

But analysts say Russia's invasion force was facing unsustainable troop and equipment losses, and that their window for a breakthrough was narrowing with Ukraine now bringing reinforcements into the fray.