The primary target of a crippling computer virus that spread from Ukraine across the world this week is highly likely to have been that country's computer infrastructure, a top Ukrainian police official told Reuters on Thursday.

Cybersecurity firms are trying to piece together who was behind the computer worm, which has paralyzed thousands of machines worldwide, shutting down ports, factories and offices as it spread through internal organizational networks to an estimated 60 countries.

Ukrainian politicians were quick on Tuesday to blame Russia, but a Kremlin spokesman dismissed "unfounded blanket accusations." Kiev has blamed Moscow for two previous cyberstrikes on the Ukrainian power grid and other attacks since Russia annexed Crimea in 2014.