President Vladimir Putin said on Friday that neighbouring Ukraine was becoming 'anti-Russia' and that Moscow would be ready to react to what he said were threats to its own security.
Putin was speaking a day after a Ukrainian court placed Viktor Medvedchuk, a prominent pro-Russian politician who says Putin is godfather to his daughter, under house arrest.
Medvedchuk, who has promoted closer ties with Moscow and acted as an intermediary between Moscow and Kyiv in the past, is being investigated over treason allegations he calls politically-motivated.
Putin, in remarks to a meeting of Russia's security council, called what was happening in Ukraine a 'cleansing' of the political space and accused the Ukrainian authorities of targeting people who favoured better ties with Russia and supported a peaceful settlement in eastern Ukraine where Ukrainian forces have been fighting Russian-backed separatists since 2014.
"Judging by everything, and this is very sad, Ukraine is slowly but surely turning into some kind of polar opposite of Russia, some kind of anti-Russia, and into a platform from whose territory it seems we will constantly receive news requiring our special attention from a security point of view," said Putin.
Referring to what he described as a selective and politically-motivated crackdown in Ukraine on certain individuals doing business with Russia, Putin said Moscow would not stand idly by.
"This is, of course, an issue that should always be on our radar and we should respond to this given the threats being created for us in a timely and appropriate manner," said Putin.
Ukrainian President Volodymyr Zelenskiy said on Friday the crackdown on Medvedchuk, which began in February when he and several associates were sanctioned and three television channels owned by an ally were forced off air, was a legal way of choking off what he described as his malign influence.
"For the first time in many years the number of oligarchs has not increased, but decreased. Minus Medvedchuk," Zelenskiy wrote in a blog.
"With the help of legal tools, Medvedchuk was stripped of the possibility of using media assets and state property to openly attack the country and damage state security."
Relations between Moscow and Kyiv collapsed after Russia annexed the Ukrainian region of Crimea in 2014 and Russian-backed separatists took control of a chunk of eastern Ukraine that same year.
Tensions have flared again in recent months after the two countries traded blame for an uptick in fighting in eastern Ukraine, and Russia, in what it called a defensive exercise, massed troops on its western border with Ukraine and in Crimea.