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 from Ukraine and was annexed by Moscow last month.

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Moscow has denied that it is behind the unrest. Sunday it denounced what it called the "use of harsh violence against protesters" by Kiev and called an urgent session of the United Nations Security Council. U.S. and Russian envoys exchanged angry words at the session, each accusing the other of meddling in Ukraine's affairs and backing violence.

Ukraine branded the gunmen terrorists and said the armed forces would be deployed to evict them. But to do so would require a show of force far greater than anything it has mustered so far.