FBI warns firms of destructive malware use by hackers
The US Federal Bureau of Investigation (FBI) has warned that cyber-hackers have used malicious software to launch destructive attacks in the US.
A five-page confidential warning was issued to US businesses on Monday, according to Reuters news agency.
The software would make it impossible to recover any lost data, the FBI said.
The warning follows a confirmation from the FBI that it is investigating last week's hack into Sony Pictures Entertainment's network.
Sony Pictures was forced to shut down its corporate network in the attack and some of its unreleased films have also been leaked online.
But the warning from the US authorities did not name any victims that have been targeted.
Instead, it provided some technical details about the malicious software and advice on how to handle such an attack.
North Korea in the spotlight
There have been accusations that North Korea may have been responsible for the attack on Sony Pictures after the country's government described a film due to be released by the studio on Christmas Day as an "undisguised sponsoring of terrorism as well as an act of war".
The Interview, starring Seth Rogan and James Franco, tells the story of a CIA plot to assassinate the North Korean leader.
Pyongyang has written letters to the UN Secretary General and President Barack Obama asking for the film to be blocked, but when asked if it was involved in the cyber-attack on Sony, a spokesman only said "wait and see".
Sony has not accused North Korea of involvement and said it was investigating the source of the hacking.