A hooded man holds a laptop computer as cyber code is projected on him in this illustration picture taken on May 13, 2017. Reuters

A massive cyberattack warning Ukrainians to "be afraid and expect the worst" hit government websites on Thursday night, leaving some websites inaccessible on Friday morning and prompting Ukraine to open an investigation.

Ukraine`s foreign ministry spokesperson said it was too early to say who could be behind the attack but said Russia had been behind similar strikes in the past.

The cyberattack, which hit the ministry of foreign affairs, the cabinet of ministers and the security and defence council among others, comes as Kyiv and its allies have sounded the alarm about a possible new military offensive on Ukraine.

Russia has previously denied being behind cyber attacks on Ukraine.

"Ukrainian! All your personal data was uploaded to the public network. All data on the computer is destroyed, it is impossible to restore it," said a message visible on hacked government websites, written in Ukrainian, Russian and Polish.

"All information about you has become public, be afraid and expect the worst. This is for your past, present and future."

The United States warned on Thursday that the threat of a Russian military invasion was high. Russia said dialogue was continuing but was hitting a dead end as it tried to persuade the West to bar Ukraine from joining NATO and roll back decades of alliance expansion in Europe - demands that the United States has called "non-starters".

"It`s too early to draw conclusions, but there is a long record of Russian assaults against Ukraine in the past," the foreign ministry spokesman said.

A top Ukrainian security official said, "All subjects of cyber security were aware of such possible provocations by the Russian Federation. Therefore, the response to these incidents is carried out as usual."