Secretary of State Hillary Rodham Clinton described Google's allegations of Chinese hacking of its Gmail system as very serious.
The Obama administration said Thursday that no official government email accounts were compromised.
Google traced the origin of the attacks to Jinan, China, the home city of a military vocational school in China. Computers at that school were linked to an assault on Google's systems 17 months ago. The two attacks are not believed to be linked.
White House spokesman Jay Carney said the Obama administration does not restrict government employees from using personal Gmail accounts, but does direct workers to use government email for official business. He had no comment on who in the administration may have been affected by the hacking.
The Pentagon said it had little information since the reported breaches involved personal accounts and not government email. Since the accounts were not official, the U.S. Department of Defense didn't know whether defense employees were among the targeted individuals.
A day after Google exposed the breach, China denied on Thursday that it supports hacking and said it is part of global efforts to combat computer security threats.
Google said all of the hacking victims have been notified and their accounts have been secured.
The Associated Press contributed to this story.