The website of Ukraine's energy and coal ministry website has been hit by a cyber attack, though the websites of state-run energy companies have not been affected, the ministry's spokeswoman said by phone.

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As of 07:34 GMT, a message posted in English on the website was demanding a ransom paid in Bitcoin to recover encrypted files, Reuters reported.

"Our specialists are working right now... We do not know how long it will take to resolve the issue," the spokeswoman said.

"Ukrenergo, Energoatom – everything is okay with their sites, it's only our site that does not work."