By Andrew Higgins and David M. Herszenhorn

New York Times News Service

DONETSK, Ukraine — As the government in Kiev moved to reassert control over pro-Russian protesters across eastern Ukraine, the United States and NATO on Tuesday 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.”

In Moscow, the Russian foreign minister, Sergey Lavrov, on Tuesday denied the accusations of Russian meddling in Ukraine. He said Russia would seek talks on the Ukrainian political crisis that could involve the United States, the European Union and “all the political forces in Ukraine,” which should include representatives of the southeastern region.

But none of that was soothing nerves rattled by days of protests here, orchestrated or otherwise. With pro-Russian demonstrators having been expelled from a government building in the eastern city of Kharkiv and the government determined to end the protests across the south and east, separatist protesters here in the east’s biggest urban center reinforced barricades outside the occupied regional administration building and vowed to stand firm, setting up a possibly violent showdown.

The operation in Kharkiv was announced by Ukraine’s acting interior minister, Arsen Avakov, who had traveled to the city to supervise the action. He wrote on Facebook that the building was retaken “without firing a shot, grenades, or other special weapons,” and that the troops were part of a broader redeployment in the region to contain unrest that Ukraine has accused Russia of orchestrating.

The Russian Foreign Ministry issued a statement in response to the use of the Interior Ministry troops, accusing Ukraine’s government of embedding nationalist militants from the group Right Sector and private U.S. mercenaries from a company called Greystone in its forces in the east. The statement said the U.S. contractors were disguised as members of a Ukrainian military unit called Falcon.

A private U.S. security company formerly affiliated with Greystone, called Academi, issued a statement in mid-March saying its employees were not working in Ukraine, after similar allegations surfaced in the Russian news media. But it was unclear what role, if any, Greystone, had in Ukraine.

The ministry, which has denounced the government in Kiev as the illegitimate product of a coup, warned against the use of military force in eastern Ukraine.

Pro-Russian demonstrators seized government buildings Sunday evening in several eastern cities, including Kharkiv, Donetsk and Lugansk, posing a challenge for the authorities in Kiev, who wrested power from the former president, Viktor Yanukovych, using similar tactics. Russian troops are deployed along the border nearby, and the Kremlin has warned that it is prepared to intervene again in Ukraine to protect the many ethnic Russians living there, as it had in Crimea in the south.