"Those in power in the Ukrainian capital should come to their senses and stop killing their citizens. Otherwise the fate of the country could turn out to be really sad," Medvedev said on Facebook.
"The use of force in Ukraine's southeast is a sign of criminal helplessness of Kiev's de facto rulers."
He accused the authorities of launching a "reprisal raid" after Kiev early on Friday launched a military assault against pro-Moscow rebels in the eastern Ukrainian town of Slavyansk.
"People are dying. Blood is being shed," Medvedev said.
More From This Section
"Responsibility for a war against their own people rests with those who are making criminal decisions in Kiev."
The term "reprisal raid" is used in Russian history textbooks to describe military operations by Nazi troops against civilians during World War II.
Today's offensive caused outrage in Moscow where President Vladimir Putin's spokesman Dmitry Peskov said it dealt a final blow to a peace deal agreed in Geneva last month.