The websites of Ukraine's government, foreign ministry and state security service were down on Wednesday in what the government said was the start of another massive denial of service (DDoS) attack that began at around 4 p.m. (1400 GMT).
Ukrainian authorities said this week they had seen online warnings that hackers were preparing to launch major attacks on government agencies, banks and the defence sector.
Ukraine has suffered a string of cyberattacks that Kyiv has blamed on Russia. Moscow, which is caught up in a mounting confrontation with the West over Ukraine, has denied any involvement.
"At about 4 pm, another mass DDoS attack on our state began. We have relevant data from a number of banks," said Mykhailo Fedorov, Minister of Digital Transformation, adding that the parliament website was also hit.