Here is the list of companies hit by hackers

Experts say the virus is a modified version of the so-called Petya ransomware, which hit last year and demanded money from victims in exchange for their data.

A similar virus called WannaCry infected more than 200,000 computers in more than 150 countries last month.

Following is a list of companies and organisations which say they have been a victim of the latest attack:

- Ukraine's central bank, the National Bank of Ukraine

- Ukrainian bank Oschadbank

- Ukrainian delivery service company Nova Poshta

- Russian state oil giant Rosneft

- Kyivenergo, Kiev power company

- Radiation monitoring system at Chernobyl

- Website of Kiev's Boryspil international airport

- Danish sea transport company Maersk

- British advertising giant WPP

- French industrial group Saint-Gobain

- US pharmaceutical giant Merck