Pro-Russian mobs have taken control of more government buildings in Ukraine, as Russian President Vladimir Putin demanded the Ukrainian government pull back its troops from the southeastern part of the country.

Watch: Protesters strip police of weapons and force them to surrender.

Ukraine's Acting President Oleksandr V. Turchynov said his country's forces are "helpless" as Russia continues to gain ground in dozens of key cities.

Watch: This Is What the US' Boldest Show of Force Toward Russia Looks Like

Fear has spread across the region, prompting hundreds of American troops to be sent for joint training in Latvia, Lithuania and Estonia, where the fears of a Russian invasion are very real.

"You've seen what has occurred in Ukraine, Crimea, and so to them it's a real, it's a real threat," said Lt. Gen. Donald M. Campbell Jr., the commanding general of the U.S. Army Europe.

Watch: What It's Like for a US Soldier Taking a Stress Test in Russia's Shadow

Latvia is not a place America's battle-hardened troops thought they would ever end up, but they arrived with 50 pounds of gear, injury training, and live fire, to train as the crisis in the area continues.

(Sergei Grits/AP Photo)

ABC News' Caterina Andreano contributed to this story.