"We have been witnessing the Russian response to the president's decree about closing access to Russian social networks," President Petro Poroshenko's deputy administration chief Dmytro Shymkiv said in a statement posted late Tuesday on Facebook.
"The president's website has sustained an organised attack." Shymkiv added that IT specialists had the situation under control and that the website was no longer under threat.
The website was accessible on Wednesday morning in Kiev. There was no immediate response to Kiev's accusation from Moscow.
IT specialists and Western governments are rarely able to pin a hacking attack directly on the Kremlin but often accuse groups or individuals close to the Russian government of being responsible for them.
Ukraine on Tuesday blocked Russia's most popular social media networks and an internet search engine in response to the Kremlin's alleged backing of a three-year separatist war in the east.
"In a single move Poroshenko dealt a terrible blow to freedom of expression in Ukraine," Human Rights Watch researcher Tanya Cooper said in a statement Wednesday. "It's an inexcusable violation of Ukrainians' right to information of their choice."
Kremlin spokesman Dmitry Peskov said Tuesday that Poroshenko's decision was "another manifestation of unfriendly, short-sighted policy toward Russia".
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