The FBI and other federal agencies are investigating, the agency said in a statement.
Postal Service spokesman David Partenheimer said the personal information that may have been obtained in the attack includes employees' names, dates of birth, Social Security numbers, addresses, emergency contacts and other information.
However, he also said that customers at local post offices or those using its website, usps.com, were not affected. But people who used its call centre may have had telephone numbers, email addresses and other information compromised. He said that the attacks happened in mid-September. Just when the breach occurred is under investigation, he said.
The agency isn't recommending that those customers take any action.
The Postal Service provided no immediate information on how many people may have been affected. It employs over 800,000 workers.
"The intrusion is limited in scope and all operations of the Postal Service are functioning normally," Partenheimer said.
"It is an unfortunate fact of life these days that every organization connected to the Internet is a constant target for cyber intrusion activity," Postmaster General Patrick Donahoe said in a statement. "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 any potential misuse of their data."
Partenheimer said the attacks affected Postal Service workers across the board, from the postmaster general to letter carriers to those who work in the inspector general's office.
Rep. Elijah Cummings of Maryland, the senior Democrat on the House Oversight and Government Reform Committee, said the panel had received two classified briefings on the attacks.
"The increased frequency and sophistication of cyber-attacks upon both public and private entities highlights the need for greater collaboration to improve data security," he wrote in a letter to Donahoe.
FBI spokesman Joshua Campbell confirmed the investigation
And while declining to discuss details of what he called "an ongoing investigation," Campbell said the FBI was working with the Postal Service "to determine the nature and scope of this incident."
"Impacted individuals should take steps to monitor and safeguard their personally identifiable information and report any suspected instances of identity theft to the FBI's Internet Crime Complaint Center at www.ic3.gov , " he said.
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