(CNN) — US cyber officials issued an “emergency directive” Thursday ordering federal agencies to defend their networks against an “advanced” group of hackers that have breached at least one agency in an apparent espionage campaign.

Government officials have not commented on who is behind the hacks, but private experts say they believe the hackers are state-backed and based in China. The hackers have been exploiting previously unknown flaws in software made by Cisco for several months.

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