Ukraine presidential aide Andriy Yermak on Friday said Russian leader Vladimir Putin is "afraid of any competition", in comments after the announcement of the death of jailed Kremlin critic Alexei Navalny.

"Putin is the ultimate evil who is afraid of any competition. The lives of Russians are nothing to him," Yermak wrote on Telegram, adding "everyone who calls for negotiations must realise that he cannot be trusted. The only language he understands is force."