There's no kill switch this time.

Security researchers said the new "Petya" cyberattack sweeping around the world might be worse than the WannaCry attack in May, and it might be harder to stop.

Ukraine first reported a cyberattack hitting its national bank, airport, and government departments, before a Russian oil company, ad giant WPP, and Dutch logistics firm Maersk all started reporting problems.

Microsoft said in a security update that the attack has hit 64 countries so far, including Russia, Brazil, and the US.

The attack targets Windows PCs and takes the form of ransomware, encrypting users' files and demanding payment in exchange for decryption. In this instance, hackers are asking for $300 worth of Bitcoin.