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US officials are worried that hackers backed by foreign governments were able to spy on communications between the US government and private companies for the past three years, following revelations of a major breach at computer network company Juniper Networks.

The breach, currently investigated by the FBI, involved hackers penetrating networks by installing backdoors on computer equipment. The breach could have allowed hackers to infiltrate any company or government network that used the equipment. It is akin to “stealing a master key to get into any government building,” said one US official.

Due to the sophistication of the attacks, it is believed that the hack was the work of a foreign government. China and Russia are among the prime suspects. US spy agencies are not implicated in the attack, according to US officials.

It is as yet unclear what classified information was compromised. Juniper Networks’ equipment is so widely used that determining the extent of the breach make take some time.

“We are aware of the vulnerabilities recently announced by Juniper. The Department of Homeland Security has been and remains in close touch with the company. The administration remains committed to enhancing our national cyber security by raising our cyber defenses, disrupting adversary activity, and effectively responding to incidents when they occur,” a senior administration official told CNN.

Juniper Networks has issued an emergency security patch and urged customers to update their systems “with the highest priority.” The patch is intended to fix the back door created by the hackers that was used to remotely login into commonly used networks.