Many Ukrainian government websites were down on Friday after a massive robbery attack, Ukrainian officials said.
While it is not clear who caused the attack, it comes amid tensions between Russia and after talks between Moscow and Western countries failed to make progress this week.
"As a result of the major terrorist attacks, websites of the Department of Foreign Affairs and other government agencies have temporarily collapsed. Our experts are already working to restore IT services," Ukrainian Foreign Ministry spokesman Oleg Nikolenko wrote on Facebook on Friday.
Nikolenko told the Associated Press that he was too close to say who could be responsible for the attack. "It is too early to reach conclusions as the investigation continues, but there is a long history of Russian cyber attacks against Ukraine in the past," he said.
Earlier Moscow had denied involvement in an online attack against Ukraine.
Websites of the country's Cabinet, seven services, the Ministry of Finance, the National Emergency Service and the government services website, which holds electronic passports and vaccination certificates for Ukrainian citizens, were not available on Friday due to the robbery.
The websites contain a message in Ukrainian, Russian, and Polish, claiming that the personal data of the Ukrainian people has been leaked to the public domain. “Fear and expect the worst. This is your past, present and future, ”the message read in part.
Ukraine's State Service of Communication and Information Protection said no personal data had been leaked.
The US estimates that Russia has recruited about 100,000 troops near Ukraine, a number that has sparked fears of an offensive. Moscow says it has no plans to attack and rejects Washington's request to withdraw its troops, saying it has the right to send them where necessary.
The Kremlin has demanded security assurances from the West that hampered NATO's rise in the east.
Last month, Moscow sent security documents demanding that NATO withdraw Ukraine and other former Soviet republics and cancel the deployment of its allies in Central and Eastern Europe. Washington and its allies have refused to provide these promises, but say they are ready for talks.
The high-profile talks this week between Moscow and the US, followed by a meeting between Russia and NATO and a meeting of the Organization for Security and Cooperation in Europe, failed to bring immediate progress.