Hackers have carried out cyber attacks on at least 10 Norwegian oil, gas and defense companies, the BBC reports.

The country's National Security Agency says that hackers sent emails with viruses attached to specific individuals. They were able to gain access to valuable company data in some cases relating to the companies' negotiations of big contracts.

According to the Associated Press it is estimated that more than one person is behind the cyber attacks. Because of the swift nature of the attacks, it is entirely possible that more companies have been targeted but are unaware of the hacking at the moment.

It is unclear exactly who is responsible for the attacks.