The websites of Ukraine's government, foreign ministry and state security service were temporarily down in what the government says was the start of a massive denial of service (DDoS) attack.
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 officials in Kyiv blamed on Russia.
Russia, which is caught up in a mounting confrontation with the US and its allies over Ukraine, has denied any involvement.
"At about 4pm, 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.
He did not mention which banks were affected and the central bank could not immediately be reached for comment.
The online networks of Ukraine's defence ministry and two banks were overwhelmed last week in a separate cyber attack.
The US company Netscout Systems Inc later said the impact had been modest.
Internet blockage observatory NetBlocks posted a tweet on Thursday that showed a drop-off in connections to the Ukrainian government websites.
"We've observed that the current network disruption has partial impact on the network layer to multiple defence and ministerial websites in Ukraine," NetBlocks director Alp Toker told Reuters.
"The spread of outages is similar but distinct from the recent DDOS attack targeting defence and banking platforms, with the latter not impacted in this instance," he added.