Russia is warning Ukraine to stop any military preparations in the south and east of the country, saying this could start a civil war.
On Sunday, pro-Russian activists had declared independence from Kiev in Kharkiv, Donetsk and Luhansk and stormed government buildings.
In a statement on its website, the Russian Foreign Ministry said Ukrainian officials planned to send Interior Ministry troops and the National Guard to restore order in the east and south, the BBC reports.
It claimed Ukrainians planned to use force to suppress the unrest and that private American military contractors were among the troops.
Ukraine's government has said it would avoid any bloodshed in dealing with the disturbances.
The United States has voiced "great concern" about heightened tension in eastern Ukraine. Secretary of State John Kerry has told Russian counterpart Sergei Lavrov in a phone call that any Russian efforts at destabilisation "would incur costs". They discussed the possibility of direct talks within the next 10 days.
Ukraine is sending security officials to Donetsk, Luhansk and Kharkiv following Sunday's unrest. In Kharkiv, about 70 separatists were arrested on Tuesday for seizing the regional administration building.
Rebels occupying Donetsk's regional government building declared a "people's republic" on Monday and called for a referendum on secession from Ukraine to be held by 11 May.
Russia recently annexed the Ukrainian peninsula of Crimea, where the majority of people are Russian speakers, following a referendum that Kiev and the West say was illegal.
Moscow now has thousands of troops massed along its border with eastern Ukraine. Although it insists it has no intention of invading Ukraine, it says it reserves the right to defend ethnic Russians in the country.
Russia is refusing to recognise the new authorities in Kiev, who took power after pro-Moscow President Viktor Yanukovych was ousted in February this year.