Kiev (CNN) -- Russia warned Tuesday that any use of force in Ukraine's eastern region could lead to civil war, as Kiev seeks to regain control after pro-Moscow uprisings in three cities.

Pro-Russian protesters seized government buildings in the three cities of Donetsk, Luhansk and Kharkiv on Sunday. Rebels occupying Donetsk's regional government building Monday declared a "people's republic" and called for a referendum on secession from Ukraine to be held by May 11.

Russia's Foreign Ministry said reports that the protesters are facing a crackdown by Ukrainian authorities are of particular concern.

"We are calling for the immediate cessation of any military preparations, which could lead to civil war," it said in a statement on its website.

The ministry alleged that what it called "American experts from the private military organization Greystone," disguised as soldiers, as well as militants from the Ukrainian far-right group Right Sector, had joined Ukrainian forces preparing for the crackdown in the country's east.