Ukraine said Sunday it had "evidence" that Russia was behind a massive cyberattack that knocked out key government websites this past week, while Microsoft warned the hack could be far worse than first thought.
Tensions are at an all-time high between Ukraine and Russia, which Kyiv accuses of having massed troops on its border ahead of a possible invasion. Some analysts fear the cyberattack could be the prelude to a military attack.
On Friday, Washington also accused Russia of sending saboteurs trained in explosives to stage an incident that could be the pretext to invade its pro-Western neighbour.
"All the evidence points to Russia being behind the cyberattack," the Ukrainian digital transformation ministry said in a statement. "Moscow is continuing to wage a hybrid war."
The ministry urged Ukrainians not to panic, saying their personal information was protected.
The purpose of the attack, it added, "is not only to intimidate society. But to also destabilise the situation in Ukraine, halting the work of the public sector and crushing Ukrainians' trust in the authorities".
The Kremlin earlier rejected the claims and said there was no evidence Russia was behind the attack.
"We have nothing to do with it. Russia has nothing to do with these cyberattacks," President Vladimir Putin's spokesman, Dmitry Peskov, told CNN.
"Ukrainians are blaming everything on Russia, even their bad weather in their country," he said in English.
Kyiv said late Friday it had uncovered preliminary clues that Russian security services could have been behind the cyberattack.