The Federal Bureau of Investigation (FBI) has warned U.S. businesses that hackers have used malicious software to launch destructive attacks in the United States.
The FBI said on December 1 that the malware overrides data on hard drives of computers, which can make them inoperable and shut down networks.
Separately, the FBI confirmed it was investigating a hacking attack at California-based Sony Pictures Entertainment last week.
It came ahead of the release of a film by Sony Pictures about North Korean leader Kim Jong Un.
Asked if it was involved in the attack, a spokesman for the North Korean government replied: "Wait and see."
In June, Pyongyang complained to the United Nations and the United States over the comedy film The Interview, describing it as an act of war.
Based on reporting by Reuters, and AP, and the BBC