Deputy PM Pavlo and other government officials couldn't access their computers.
It is a suspected ransomware that is demanding a payment of $300 in Bitcoin to regain access to files.
Reportedly, some organizations in India have been affected too.
Massive cyberattack in Ukraine
Banking operations, customer services affected in Ukraine
Rozenko Pavlo said all the government systems are down; Ukraine's national bank blamed an "unknown virus".
Pavlo posted his computer's picture, which displayed a message that a disk "contains errors and needs to be prepared" and warned not to turn off the PC.
Ransomware 'Petya' similar to Wannacry
According to experts, it could be an attack by ransomware called 'Petya', which emerged last year, and its updated version 'Petrwrap'.
They say the method of virus distribution is similar to that of WannaCry, which affected over 150 countries recently.
Companies are being locked out and a payment for decryption of files is being demanded, just like WannaCry.
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28 Jun 2017
Ransomware attack: India's shipping industry affected as Mumbai port hit
As a result, operations at the Gateway Terminals India are being carried out manually. GTI is operated by AP Moller-Maersk, the world's biggest shipping company, which is among Petya's victims.
Hague-based APM Terminals, which operates Gujarat's Pipavav terminal, has also been affected.
Petya takes down large businesses around the world
Globally, several large businesses have been hit. Russia's biggest oil company Rosneft, the world's biggest shipping firm Maersk and advertising giant WPP Group are some of Petya's victims. Ukraine's central bank and an international airport were affected. Attacks were reported in Europe and England too.
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