"President Putin's aim seems to be to turn Ukraine into a failed state and to suppress and discredit alternative voices in Russia," NATO Deputy Secretary General Alexander Vershbow said.
"We've seen that the victims are not just in eastern Ukraine, with the brutal murder of Boris Nemtsov last Friday," Vershbow told members of parliament from EU countries at a conference in Riga.
Putin said on Wednesday that the killing of Nemtsov, a former deputy prime minister who was shot dead last week, was a shameful tragedy. The Kremlin denies any involvement, saying that the killing was a "provocation" to discredit Putin.
"While we don't know who pulled the trigger, we do know that Boris Nemtsov was a powerful voice for democracy and against Russia's involvement in Ukraine ... (and) was among those vilified as "traitors" and "fifth columnists" in Russia's official propaganda," Vershbow said.
Since last summer, reports have been circulating inside the country that many serving Russian troops have died in combat in eastern Ukraine, where over 5,600 people have been killed in a pro-Russian insurgency.
Moscow denies sending arms or troops to the region, saying any Russians fighting in Ukraine are volunteers.
