DONETSK, Ukraine — As the government in Kiev moved to reassert control over pro-Russian protesters across eastern Ukraine, the United States and NATO issued stern warnings to Moscow about further intervention in the country’s affairs amid continuing fears of an eventual Russian incursion.

Secretary of State John Kerry accused the Kremlin of fomenting the unrest, calling the protests the work of saboteurs whose machinations were as “ham-handed as they are transparent.” Speaking to the Senate Foreign Relations Committee, he added: “No one should be fooled — and believe me, no one is fooled — by what could potentially be a contrived pretext for military intervention just as we saw in Crimea. It is clear that Russian special forces and agents have been the catalysts behind the chaos of the last 24 hours.”

The secretary general of NATO, Anders Fogh Rasmussen, said Russia would be making a “historic mistake” by going into Ukraine, and he urged the Kremlin to “step back.” At a news conference in Paris, he said any such actions “would have grave consequences for our relationship with Russia” and “would further isolate Russia internationally.”