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"In breaking the ceasefire, (Ukraine's) President (Petrol) Poroshenko made a dramatic mistake. It will cause more casualties. And he is now personally responsible for them," Medvedev wrote on his Facebook page.
"It will be much more difficult to revive talks. These are the rules of a war," he said, referring to talks between the Kiev government and separatist, pro-Russian rebels in eastern Ukraine.
Medvedev wrote his comments before the results were known of talks in Berlin among the foreign ministers of Russia, Ukraine, Germany and France. The Russian and Ukrainian ministers agreed to hold three-way talks with the rebels by Saturday, in an effort to establish an new ceasefire.
Medvedev also reiterated that Russia would introduce measures to "protect" its economy and mount barriers to trade with Ukraine after Kiev signed a historic association agreement that brought it closer to the European Union on July 27.
"Given the circumstances, it will extremely difficult to develop ties with Ukraine, impossible in some areas," he wrote.