Companies across the globe are reporting that they have been struck by a major ransomware cyber-attack.British advertising agency WPP is among those to say its IT systems have been disrupted as a consequence.
The virus, the source of which is not yet known, freezes the user's computer until an untraceable ransom is paid in the digital Bitcoin currency.
Ukrainian firms, including the state power company and Kiev's main airport, were among the first to report issues.
The Chernobyl nuclear power plant has also had to monitor radiation levels manually after its Windows-based sensors were shut down.Interpol involvement
In a statement, the US National Security Council said government agencies were investigating the attack and that the US was "determined to hold those responsible accountable".
The US Department of Homeland Security advised victims not to pay the ransom, saying there was no guarantee that access to files would be restored.
The Russian anti-virus firm Kaspersky Lab said its analysis showed that there had been about 2,000 attacks - most in Ukraine, Russia and Poland.
The international police organisation Interpol has said it was "closely monitoring" the situation and liaising with its member countries.
Experts suggest the malware is taking advantage of the same weaknesses used by the WannaCry attack last month.
"It initially appeared to be a variant of a piece of ransomware that emerged last year," said computer scientist Prof Alan Woodward.
"The ransomware was called Petya and the updated version Petrwrap.
"However, now that's not so clear."
Kaspersky Lab reported that it believed the malware was a "new ransomware that has not been seen before" despite its resemblance to Petya.
As a result, the firm has dubbed it NotPetya. Kaspersky added that it had detected suspected attacks in Poland, Italy, Germany, France and the US in addition to the UK, Russia and Ukraine.
Andrei Barysevich, a spokesman for security firm Recorded Future, told the BBC such attacks would not stop because cyber-thieves found them too lucrative.