Hackers have stolen data from corporate computers in Norway's oil and gas and defense industries, which have been targets of at least 10 cyber attacks over the last year.
The computer break-ins were reported Thursday by the country's National Security Authority, which did not disclose the names of the companies or details on the information taken.
Viruses written differently for each target to avoid anti-virus detection were sent by e-mail to people and companies that were in major deal negotiations, according to media reports. Once deployed, the malware scoured the recipients' computer for documents, drawings, user names and passwords. Data was stolen from some companies and sent outside the country, the NSA said.
At least 10 attacks have occurred over the last year. Because not all hacks are noticed or reported, the number could be higher, according to the NSA. Norway's oil and gas industry ranks the third largest in the world, producing 2.8 million barrels per day.
A year ago, the web site of the Norway-based Nobel Peace Prize suffered a cyber attack after it awarded the prize to imprisoned Chinese dissident Liu Xiaobo. The timing led some security experts to suspect the attack originated from China.
European companies were among the targets of Duqu, a Trojan reported last month that security experts believe was distributed to gather information as part of a cyber-espionage operation. Duqu, which was found in the computer systems of suppliers to industrial factories, has many code similarities to the Stuxnet worm, which led some experts to argue they were developed by the same team.
Discovered in 2010, Stuxnet is believed to have damaged control systems in Iran's nuclear facility. Nearly 60 percent of the infected systems worldwide were in Iran, leading experts to believe the country was the primary target.
