A man holds a laptop computer as cyber code is projected on him in this illustration picture taken on May 13, 2017. REUTERS/Kacper Pempel/Illustration

KIEV (Reuters) - Hackers took down the website of the Ukrainian energy and coal ministry on Tuesday, posting a message in English demanding a ransom paid in Bitcoin to recover encrypted files.

The attack appeared to be an isolated incident with no other government websites or systems affected, cyber police spokeswoman Yulia Kvitko wrote in a message to Reuters.

Hackers have repeatedly targeted Ukrainian government systems and critical infrastructure such as energy companies. Kiev typically blames attacks on Russia, though Moscow denies involvement.

“This case is not large-scale. If necessary, we are ready to react and help,” Kvitko said.

The websites of state-run energy companies have not been affected and the ministry’s email system was working normally, the ministry spokeswoman said by phone.

“Our specialists are working right now ... We do not know how long it will take to resolve the issue,” she said. “Ukrenergo, Energoatom - everything is okay with their sites, it’s only our site that does not work.”