Ukraine has alleged that Russian security services were involved in launching a massive cyber attack which locked up computers across the world.
The Ukrainian security agency, known as the SBU, said similarities between the malicious software and previous attacks on Ukrainian infrastructure shows Russian intelligence services took part.
Ukraine was one of countries hardest hit by the attack on Tuesday, when computers at government agencies, energy companies and cash machines were temporarily disabled as their data was encrypted amid demands for ransom payments.
Russian companies, including the state-owned oil giant Rosneft, also said they were hit by the attack.
Relations between Russia and Ukraine have collapsed since Moscow annexed Crimea in 2014 and began backing separatists fighting forces loyal to Kiev in eastern Ukraine.
Most of the organisations affected by the attack recovered within 48 hours.
The SBU said the attackers appeared uninterested in making a profit from the ransomware program and were more focused on sowing chaos in Ukraine.
There was no immediate official response from the Russian government, but Russian lawmaker Igor Morozov said the Ukrainian charges were "fiction" and that the attacks were likely to be the work of the United States.
Ukraine has repeatedly accused Russia of sponsoring hacking attacks, including the hack of Ukraine's voting system ahead of the 2014 national election and an assault that knocked its power grid offline in 2015.