More than 40 organisations have been targeted, including the US Energy, Defense, State and Homeland Security departments.
Software giant Microsoft also confirmed it has been affected.
The hacking operation was uncovered last week, but investigators believe it has been going on for months.
US officials suspect it is carried out by Russian hackers, and warned of the 'grave' risk it poses to government and private networks.
What does it mean for America's national security?
And how will Washington respond?
Labour's opposition day motion calling on the government not to drop the £20 uplift in Universal Credit has just passed in...