Hackers have exploited the computer networks of the US Postal Service, compromising the data of more than 600,000 employees and customers.
The data - which has been recorded as nearly 800,000 records - included names, date of births, social security numbers, addresses and employment dates.
The breach affected customer data that included names, email addresses and phone numbers.
Postmaster general, Patrick Donahoe said: "It is an unfortunate fact of life these days that every organisation connected to the internet is a constant target for cyber intrusion activity.
"The United States Postal Service is no different. Fortunately, we have seen no evidence of malicious use of the compromised data and we are taking steps to help our employees protect against misuse of their data."
Some analysts say the breach should be considered as the latest in a series of cyber attacks aimed at the US government.
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