Unknown hackers stealing EU files for past five years

It also hit Australia, Iran, Israel, Russia and the US, among others.

But Belgium, the home of the EU and Nato headquarters, saw 15 separate breaches - the fourth highest number of any country on the list.

Over the past five years, the hackers pulled material, such as files, as well as keystroke history and Internet browsing history, from desktop and laptop computers, servers and USB sticks.

They also stole contact lists, call history and SMS-es from iPhone, Nokia and Windows Mobile smartphones.

In some cases, they hunted for files with extensions "acidcsa, acidsca, aciddsk, acidpvr, acidppr, acidssa," which "appear to refer to the classified software 'Acid Cryptofiler,' which is used by several entities such as the European Union and/or Nato," Kaspersky Lab said in its report.

They even accessed files which had been deleted by users and used malware which quietly resurrects itself after it has been discovered.

The hackers hid behind "proxy" servers in Austria, Germany and Russia.

But Kaspersky Lab's analysis of the malicious code shows traces of Chinese and Russian-speaking authors.

"Currently, there is no evidence linking this with a nation-state-sponsored attack. The information stolen by the attackers is obviously of the highest level and includes geopolitical data which can be used by nation states. Such information could be traded in the underground and sold to the highest bidder, which can be of course, anywhere," it noted.

It added that it began its investigation in October last year following a tip-off from an anonymous "partner."

In an unusual constellation in diplomatic terms, the firm thanked cyber security officials from Belarus, Romania and the US for helping it to nail down details.