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.
