Earlier on Wednesday, the websites of several Ukrainian banks and government departments became inaccessible.
At the same time a new "wiper" attack, which destroys data on infected machines, was discovered being used against Ukrainian organisations.
The incident represents the third wave of attacks against Ukraine this year, and the most sophisticated to date.
The latest attack began on Wednesday afternoon when internet connectivity company NetBlocks tweeted about the outages, saying "the incident appears consistent with recent DDoS attacks".
Distributed denial of service (DDoS) attacks are designed to knock a website offline by flooding it with huge amounts of requests until it crashes.