Ukrainian President Petro Poroshenko’s official website has been hit by an “organised” cyber-attack from Russia, his administration has said.
It said it was Russia’s response to Mr Poroshenko’s decree banning some of Russia’s biggest media networks and internet services popular in Ukraine.
It said despite the attempt by Russian-based sites to take down servers the situation “is under control”.
Kiev provided no evidence of Russia’s involvement in the attack.
Moscow has so far made no public comments on the latest developments.
Ukraine’s decision earlier on Tuesday to impose the ban on Russian sites is a significant ramping up of sanctions on Russia for its annexation of the southern Crimea peninsula and the continuing conflict in eastern Ukraine.
Those targeted include social networks VK.com and Odnoklassniki, search engine Yandex and the Mail.ru email service.
Ukrainian service providers have been ordered to block access to the sites.
The companies’ offices in Ukraine will also face asset freezes and other restrictions although it was not immediately clear how the ban on the services would come into force and whether Ukraine had the technical means to enforce it.
The president’s decision to push through a decision by Ukraine’s national security council could prove controversial domestically, as VK (VKontakte) alone has an estimated 15 million users in Ukraine and Odnoklassiki (which translates as Classmates) is also widely popular.
Russia’s foreign ministry condemned the ban, accusing Ukraine of trying to create an “authoritarian state”.
In a separate development, Ukraine’s parliament on Tuesday banned the public display of the orange-and-black St George’s ribbon, which is seen in Russia as a symbol of Soviet victory in World War Two.
Andriy Parubiy, the speaker of the Ukrainian parliament, said the ribbon was a “symbol of Russia’s occupation war against Ukraine”.
Ukraine president’s site ‘attacked by Russia’