The Mandiant report comes a week after U.S. President Barack Obama issued a long-awaited executive order aimed at getting the private owners of power plants and other critical infrastructure to share data on attacks with officials and to begin to follow consensus best practices on security.

Both U.S. Democrats and Republicans have said more powerful legislation is needed, citing Chinese penetration not just of the largest companies but of operations essential to a functioning country, including those comprising the electric grid.

(Additional reporting by Michael Martina and Koh Gui Qing in BEIJING, Carlos Barria in SHANGHAI and Jim Finkle in BOSTON; Editing by Robert Birsel and Sanjeev Miglani)