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Russia claimed Tuesday that 150 specialists from an American private military group are in eastern Ukraine, the region where pro-Russia protesters are clashing with government forces.

A woman who identified herself as an employee of the group, the security contracting company Greystone, declined comment. She also declined to provide her name.

Greystone, on its website, identifies itself as a “provider of aviation and protective support services and training” formerly part of Xe Services, the security contracting company once known as Blackwater.

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A statement from the Russian Foreign Ministry said that the Americans were wearing the uniforms of Sokol, a Ukrainian police force, and were “of particular concern.”

A State Department official referred questions to the company and said: “As for the United States, we do not have any U.S. military units in Ukraine.”

Russia also said that militants from Right Sector, a hard-line Ukrainian nationalist organization, were trying to use force to stop protesters opposed to the new Ukrainian government in Kiev.

Pro-Russian activists have holed up in Ukrainian government buildings this week in the country’s east and have called for a referendum on breaking from Ukraine, similar to a vote last month in which the Crimean peninsula voters to leave Ukraine and join Russia.

Russia has said it has no designs on invading eastern Ukraine. The White House on Monday warned Russia against moving “overtly or covertly” into that region and said there was evidence that the pro-Russian demonstrators are being paid.

Secretary of State John Kerry said Tuesday that Russian agents are to blame for unrest this week in Ukraine, and he warned that Russia may be laying the groundwork for a military intervention, just as it did before seizing the Crimean peninsula.

— Albina Kovalyova and Erin McClam