A new strain of ransomware, known as "Bad Rabbit", has hit computer systems in Russia, Ukraine and elsewhere.
The malware affected the Russian Interfax news agency and caused flight delays at Ukraine's Odessa airport.
The ransomware is a type of virus that locks infected computers and asks victims to pay a ransom to restore access.
US officials have said they "received multiple reports of Bad Rabbit ransomware infections in many countries around the world" and urged people not to pay the ransom demanded.
Robert Lipovsky, a researcher with cyber firm ESET, said the attacks were disturbing because attackers quickly infected critical infrastructure, including transportation operators, indicating it was a "well-coordinated" campaign.
Bulgaria, Turkey and Japan have also reported instances of the malware, which bears similarities to the WannaCry and Petya outbreaks earlier this year.
They resulted in what some economists estimated are billions of dollars in losses.
The "WannaCry" ransomware attack in May forced the closure of hospitals, factories and other facilities around the world for days.