The U.S. Department of State expects its main unclassified email system to be back in operation later Monday after security upgrades, but wider Internet access could take longer to get back online.
The department, which says it fights off "thousands" of hacking attacks each day, took its system offline over the weekend "to ensure the integrity" of the network.
"It was about further enhancing our security capabilities," State Dept. spokeswoman Jen Psaki said at a regular briefing on Monday.
She said it would take some time for the entire Internet system to be back online at the government department, but email would be the first step and is expected to return on Monday night.
The security upgrade isn't in reaction to a recent compromise of the network, the department said.
In November, hackers reportedly did manage to break into the network. Their identity was never confirmed, but they were suspected to be Russian state-sponsored hackers, according to the news reports.
"The recent increase in news reports regarding cyberincidents reflects that the department is among a growing list of public institutions and private industries facing an increasing number of sophisticated cyberthreats," the department said in a statement on Friday.
The unclassified network is separate from the "ClassNet" network that handles classified information.
Martyn Williams covers mobile telecoms, Silicon Valley and general technology breaking news for The IDG News Service. Follow Martyn on Twitter at @martyn_williams. Martyn's e-mail address is email@example.com