Ukraine's President Petro Poroshenko has imposed a ban on Russia's biggest social media networks and search engines popular among millions of Ukrainians.

The measure is a significant escalation of sanctions imposed on Russia in response to its annexation of Crimea. Earlier this year, subsidiaries of Russian banks in Ukraine were sanctioned. The internet companies have been added to a blacklist of more than 400 Russian firms.

The ban, which targets social networks and Odnoklassniki, search engine Yandex and email service, comes a day after Poroshenko warned on national television that Russia was planning on launching a "cyber war" in Ukraine.

"Russia used cyber warfare trying to influence the results of elections across Europe, and in the US. We expect that they will try to do the same thing Ukraine," the Ukrainian president said in a televised address on Monday (15 May) night.

"I am confident that our determination and unity will not leave them any chance [for this]," he added.

The decision could prove controversial as VK (VKontakte) has more than 15 million users in Ukraine. It was the second most-visited website in March, according to the Ukrainian Internet Association. The other blacklisted websites also came in the top five that month.

The Kiev government's Security and Defence Council said in a statement that the ban was introduced to protect against companies "whose activities threaten the information and cyber security of Ukraine".

Service providers have been told to block access to Russian sites. The Russian companies will also have their assets frozen. It is unclear when the ban will come into force as the websites were still accessible on Tuesday afternoon.

Maria Zakharova, a spokesperson for the Russian Foreign Ministry, told Russian newspaper Kommersant that the ban was a "manifestation of politically motivated censorship".