"We call for the immediate cessation of any military preparations, which are fraught with the risk of unleashing civil war," the Russian Foreign Ministry said in a Tuesday statement.
The warning came after pro-Russia activists seized state buildings in the eastern cities of Kharkiv, Lugansk and Donetsk, where they also declared independence and vowed to vote on joining Russia.
Kiev accused Russia of fomenting the unrest and Washington warned the Kremlin to stop efforts to "destabilize Ukraine," accusations that Moscow brushed off.
The Russian foreign ministry said on Tuesday it had information that Ukraine was sending internal security forces and volunteers from its National Guard including fighters from Pravy Sektor (Right Sector) ultra-nationalist group, to southeastern Ukraine including Donetsk.
It also alleged that Ukraine was deploying US private security operatives dressed as Ukrainian special forces. It said the mercenaries came from the Greystone Ltd security firm.
It said Ukraine had tasked the forces with "suppression using force of the residents of the southeast of the country against the policies of the current Kiev authorities."
"The organizers and participants in this provocation are taking on a huge responsibility for creating a threat to the rights, freedoms and lives of peaceful Ukrainian citizens and to the stability of the Ukrainian state" it warned.
The ministry released the statement on its website following talks late Monday between Russian Foreign Minister Sergei Lavrov and his Ukrainian counterpart Andriy Deshchytsya.
The ministry said in a statement that Lavrov stressed the "necessity of a respectful attitude to the aspirations of the inhabitants of southeastern Ukraine."
Lavrov said Kiev must not allow "attempts to react by force to their legal demands for their linguistic, cultural and social-economic rights."
Lavrov called for Kiev to take "urgent measures" to organize a national dialogue, saying it was ready to "support this process along with the European Union and the United States."
BA/BA