Ukraine has issued a formal demand asking Russia to explain the necessity of holding military drills in the region bordering the two countries amid tensions.
Asking Moscow to respond within 48 hours, Kiev said the military exercises are Russia's latest attempts to undermine Ukraine's efforts in fighting the separatists in the eastern region.
The Ukrainian foreign ministry said in a statement: "Conducting Russian exercises close to the Ukrainian border cannot but raise concerns, especially after the recent declaration by the Russian Foreign Minister, containing an unambiguous hint about the possibility of Russian troops entering Ukrainian territory.
"Pursuant to provisions of the Vienna document on trust-building measures and security, Ukraine has issued a request to Russia through the Organization for Security and Cooperation in Europe (OSCE) to provide explanations for the objectives and place of the military exercise, the date on which it is expected to wind up, as well as the number and the types of Army unity involved in them."
Moscow had earlier said it was pressing ahead with its exercises in order to halt the "military machine".
"The order to use force against civilians has already been given, and if this military machine is not stopped, the amount of casualties will only grow. War games by Nato in Poland and the Baltic states are also not helping the normalisation of the situation. We are forced to react to the situation," Defence Minister Sergei Shoigu told a Russian officials' meeting.
Russian President Vladimir Putin has also warned Ukrainian authorities not to use force against their own citizens.
During an address to a media forum in St Petersburg, Putin said: "If the current Kiev authorities really started to use army against people inside the country, this is a very serious crime against their own people."