United States officials have said that the country is facing a "dedicated adversary" to the cyber security following a major breach.
According to the BBC, the officials accused China of the attack while the latter has denied any involvement.
Four million former and current US employees are being advised to take precautions.
They have been asked to freeze credit reports, monitor or close bank accounts and change online passwords.
Bryan Sivak, a former technology officer with the Department of Health and Human Services said that he was concerned about "identity theft".
A White House spokesman said that the identity of the hackers was the focus of an FBI investigation that is under way.