More than a dozen Ukrainian government websites went down on Friday, in a cyber-attack that also targeted embassies.
The foreign and education ministries were among those hit, along with embassies in the UK, US and Sweden.
Before the sites went down a message appeared warning Ukrainians to "prepare for the worst".
It was unclear who was behind the attack but a spokesman said previous cyber-attacks had come from Russia.
Russia has not yet commented. However, Ukraine's government has come under intense pressure from its neighbour, with a build-up of some 100,000 Russian troops near its borders.
Ukraine's SBU security service says in just nine months last year it "neutralised" 1,200 cyber-attacks or incidents.
At the start of Friday's attack, a message on the hacked websites was posted in three languages, Ukrainian, Russian and Polish.
"Ukrainian! All your personal data has been uploaded onto the public internet," the message read. It continued: "This is for your past, your present and your future."
The SBU in Kyiv said later that no personal data had been leaked, according to initial assessments, and no content had been changed.
Among the sites targeted was the Diia website, a key system containing government services that stores personal vaccination data and certificates.
The European Union's foreign policy chief, Josep Borrell, said all its resources were being mobilised to help Ukraine deal with "this type of cyber-attack".
A number of sites were suspended to prevent the attack from spreading, the government said, while the security service said many had already been restored.