The software giant's Threat Intelligence Center says the Nobelium group is attempting to gain access to customers of cloud computing services and other IT service providers.

The European Banking Authority warns that personal data could have been accessed as a result of the attack.

The campaign is said to have exploited recently discovered flaws in Microsoft Exchange software, stealing emails and infecting computer servers with tools that let attackers take control remotely.

So far officials have said the hackers broke into computers at the State Department, Commerce Department, Treasury, Homeland Security Department, and the National Institutes of Health.