Russia should recognize Ukraine's president-elect Petro Poroshenko and convince militants in Ukraine's southeast to lay down their weapons, U.S. President Barack Obama has said.

"Russia needs to recognize that president-elect Petro Poroshenko is the legitimately elected leader of Ukraine and engage the government in Kyiv," he said at a press conference following the G7 summit in Brussels on Thursday.

Given Russia's influence over the militants in Ukraine, it continues to have a responsibility "to convince them to end violence, lay down their weapons and enter into a dialog with the Ukrainian government," the U.S. president said.

On the other hand, if Russia's provocations continue, "the G7 nations are ready to impose additional costs on Russia," Obama said.