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.