"What has been building up for the Ukrainians is fundamentally a manpower issue," a former senior Pentagon adviser told Newsweek.

Days before the leaders of Russia and the U.S. hold a summit meeting in Alaska, Moscow’s forces breached Ukrainian lines in a series of infiltrations this week in the country’s industrial heartland of Donetsk.

Russian forces have made a sudden thrust into eastern Ukraine near the coal mining town of Dobropillia, a move that may be an attempt to increase the pressure on Kyiv to give up land as the U.S. and Russian presidents prepare to meet.