Russia and the West exchanged warnings Tuesday that the escalating tensions in eastern Ukraine could lead to civil war.
The Ukrainian government launched an "anti-terrorism" operation Monday night in the Kharkiv, one of three cities where hundreds of pro-Russia demonstrators seized government buildings hours earlier.
Police sealed off the city center and arrested some 70 people for "illegal activity related to separatism, the organization of mass disorder, damage to human health" and breaking other laws, Interfax Ukraine reported.
Acting Ukrainian President Oleksandr Turchynov said demonstrators used weapons to resist the police and two officers were injured by grenades.
The operation was in response to pro-Russian demonstrators seizing government buildings in Kharkiv, as well as the eastern cities of Luhansk and Donetsk on Sunday night. They were demanding a referendum for independence in a bid to follow Crimea's annexation by Russia last month.
Eastern Ukraine has a significant proportion of Russian speakers and people who identify themselves as ethnic Russians. But the White House hinted Monday it believes the protests were sponsored by the Kremlin.
Press Secretary Jay Carney told a briefing there was "strong evidence that some demonstrators were paid and not local residents." Asked if they were paid by Russia, he added that it "at least suggests that outside forces, not local forces, were participating in the effort to create this provocation."
Ukrainian Prime Minister Arseny Yatseniuk told his Cabinet Tuesday that "an anti-Ukrainian plan is being put into operation ... under which foreign troops will cross the border and seize the territory of the country," according to a Reuters report.
NATO Secretary General Anders Fogh Rasmussen also told a conference in Paris on Tuesday that Russia should scale back what he said were "tens of thousands of troops" on Ukraine's borders.
The Russian foreign ministry warned Ukraine over massing its own forces. "We call for an immediate halt to military preparations which could lead to an outbreak of civil war," the ministry said in a statement.
Reuters contributed to this report.