Ukraine said on Friday it had been hit by a "massive cyber attack" that knocked out government websites and posted menacing messages in their place.
The attack prompted Nato to offer cyber-security assistance to Ukraine amid suspicion from European diplomats that Russia was behind the attack.
While the Ukrainian government did not immediately assign blame for the overnight hack, which brought down several websites including the Foreign Ministry homepage, it comes amid a tense military stand-off with Russia which has previously been accused of cyber meddling in Ukraine.
Russia has signalled it is losing patience with peace talks and said on Friday that it expected a written answer by next week on its proposals to curb Nato expansion.
"Ukrainians! All your personal data was uploaded to the public network," said a message on the hacked websites written in Ukrainian, Russian and Polish, which later appeared to have vanished.
"All information about you has become public, be afraid and expect the worst. This is for your past, present and future."
The government contradicted this threat by saying that no personal data had been leaked. The websites of Ukraine's Foreign Ministry, Cabinet of ministers, Security and Defence Council and Ministry of Education all appeared to be down after the attack.
Oleg Nikolenko, a Foreign Ministry spokesman, said a specialist cyber-police department was investigating what he called a "massive cyber attack".
It comes as tension soars between Russia and the West in the shadow of Moscow's military build-up on Ukraine's eastern border. The Kremlin's peace terms, which include blocking any expansion of Nato, are rejected by the US and its allies.
Nato Secretary-General Jens Stoltenberg issued a statement "strongly condemning" the attack, and said alliance cyber experts in Brussels were helping their Ukrainian counterparts.
Ukraine and Nato will sign an agreement on greater cyber co-operation in the coming days, said Mr Stoltenberg, including granting Ukraine access to a malware information sharing platform.
European diplomats, while stopping short of accusing Russia outright, made clear their suspicions that Moscow was involved.
Josep Borrell, the EU's leading foreign policy representative, said there was no proof of who was behind the attack but added: "We can imagine".
Ann Linde, Sweden's Foreign Minister, said at the start of EU talks on Friday that the cyber attack was "exactly one of the things we have warned of and we are afraid of" as tension escalates.
"Of course, we have to see who is responsible for it," she said. "It's also something that is heightening the tension, and that's why we have to be very firm in our messages to Russia that if there are attacks against Ukraine, we will be very harsh and very strong and robust in our response."
The West has accused Russia of stationing tanks, artillery and about 100,000 soldiers on Ukraine's war-torn eastern border in recent weeks, in what Nato suspects is preparation for an invasion. Moscow says it has no plans to invade Ukraine.
The stand-off with Russia "is serious, more serious than anything we've seen in recent years", Austrian Foreign Minister Alexander Schallenberg said.
"Some say the cyber-attack could be the prelude for other activities, military activities," he said.
The US and its Nato allies this week held talks with Russia in an attempt to ease tension but all three rounds of negotiations, in Geneva, Brussels and Vienna, ended without a breakthrough.
Russian Foreign Minister Sergey Lavrov said on Friday that Moscow would not wait indefinitely for the West to hear its demands to limit Nato expansion.
“We have run out of patience,” Mr Lavrov said at a news conference. “The West has been driven by hubris and has exacerbated tensions in violation of its obligations and common sense."
Russia separately announced a snap combat readiness inspection of troops in its far east and said they would practise deploying to faraway military sites for exercises.