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 and demands an untraceable ransom be 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.
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.