The attacks focussed on research and academic organizations, one of which was TeraGrid, a government-funded project to join up several supercomputers.
The University of Illinois National Centre for Supercomputing Applications, Stanford University and the National Centre for Atmospheric Research were also hit.
In a statement, the Atmospheric Centre said its computers were compromised and some of its network was taken offline.
Officials tried to dampen speculation over the severity of the threats, but IT security experts said that such a break-in could lead to a serious attack on the internet.
