SLOVYANSK, Ukraine—Pro-Russian activists and militants extended their grip across eastern Ukraine, prompting the government to mobilize the military as it struggled to prevent a replay of Russia's takeover of Crimea.

The U.S. and its Western allies said some of the well-equipped gunmen who participated in seizing police stations and other buildings in several cities and towns over the weekend appeared to be Russian special forces in unmarked uniforms. Such troops moved into Crimea shortly before the region voted to secede...