Russian President Vladimir Putin has vowed to respond in a timely fashion to a move by Ukraine to prosecute a Ukrainian lawmaker and close ally of Moscow and put him under house arrest in a case of alleged treason.
"Taking into account all the threats that are being created for us, we will have to respond to this properly and in a timely fashion," Putin said at a meeting of his security council on Friday.
He said that Kiev was clearly carrying out a "clean-up of the political landscape."
On Thursday, a court in Ukraine placed under house arrest business tycoon and lawmaker Viktor Medvedchuk.
Kiev accused him of attempting to steal natural resources from Crimea and of handing Ukrainian military secrets over to Russia.
Medvedchuk said the charges against him were politically motivated, describing them as punishment for his political viewpoints.
Putin also said, "The decisions are clearly political and selective."
"For political reasons, some are selectively blamed for working with Russia in the economic sphere, although many, including those of the country's top political leadership, have for many years been actively working in Russia," said the Russian president.
Tensions recently escalated between Ukraine and Russia.
Ukraine last month accused Russia of amassing 100,000 troops on their shared border and claimed that ethnic Russians in eastern Ukraine were systematically violating a ceasefire.
The Kremlin said at the time that the Russian troop and military hardware movements near the border with Ukraine were aimed at ensuring Moscow’s own security and posed no threat to anyone.
Kiev also accuses Moscow of supporting the ethnic Russians in the eastern Ukrainian region of the Donbass, where an armed conflict has been going on since seven years ago. Russia denies that allegation.
Siding with Ukraine, the United States and NATO have also been taking increasingly hostile postures toward Russia in recent months, citing an alleged Russian threat to Ukraine.