President Vladimir Putin called the killing of east Ukraine separatist leader Alexander Zakharchenko on Friday a "dastardly" act that aimed to destabilize the fragile peace in the region.
However, Putin, in a statement issued by the Kremlin, did not accuse Ukraine of being behind the killing. Putin expressed his condolences to Zakharchenko's family.
Russia's state investigative committee has said it was treating the killing of Zakharchenko as an act of international terrorism.
A criminal investigation into the killing has been opened, the committee said in its statement.