The FBI, Department of Homeland Security and other US authorities are investigating a massive hack into federal systems that may have compromised the personal information of up to four million current and former government employees, ABC News reported.

A US official said the hack could impact workers in several federal agencies and that what's currently known is just "the tip of the iceberg."

“The FBI is working with our inter-agency partners to investigate this matter,” an FBI spokesman said in a statement.

“We take all potential threats to public and private sector systems seriously, and will continue to investigate and hold accountable those who pose a threat in cyberspace," he said.