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Oil, gas and defence firms in Norway have been hit by a series of sophisticated hack attacks.

Industrial secrets and information about contract negotiations had been stolen, said Norway's National Security Agency (NSM).

It said 10 firms, and perhaps many more, had been targeted in the biggest wave of attacks to hit the country.

Norway is the latest in a growing list of nations that have lost secrets and intellectual property to cyber thieves.

The attackers won access to corporate networks using customised emails with viruses attached which did not trigger anti-malware detection systems.

The NSM said the email messages had been sent to specific named individuals in the target firms and had been carefully crafted to look like they had come from legitimate sources.

Many of the virus-laden emails were sent while the companies were in the middle of negotiations over big contracts.

It said user names, passwords, industrial drawings, contracts and documents had been stolen and taken out of the country.

The NSM believes the attacks are the work of one group, based on its analysis of the methods used to target individuals, code inside the viruses and how the data was extracted.

The agency said it was publishing information about the attacks to serve as a warning and to encourage other targeted firms to come forward.

"This is the first time Norway has revealed extensive and wide computer espionage attacks," the NSM said in a statement.

It said it found out about the attacks when "vigilant users" told internal IT security staff, who then informed the agency.

However, the NSM said, it was likely that many of the companies that had been hit did not know that hackers had penetrated their systems and stolen documents.

Security firms report that many other nations and industrial sectors have been targeted by data thieves in recent months.

The chemical industry, hi-tech firms and utilities appear to have been singled out.