A hacker, or hacking group, published via Twitter what they said were records of 9,000 DHS employees.
According to technology news site Motherboard, the hacker has said he will soon share the personal information of 20,000 DoJ employees, including staff at the FBI.
The news site said it had verified small portions of the breach, but also noted that some of the details listed appeared to be incorrect or possibly outdated.
In a statement, the DHS told journalists: "We take these reports very seriously, however there is no indication at this time that there is any breach of sensitive or personally identifiable information."
The Department of Justice also downplayed the breach's significance.
The hacker is understood to have used simple human engineering to bypass one stage of the authorities' security systems.
Motherboard quoted the hacker, who explained: "So I called up, told them I was new and I didn't understand how to get past [the portal].
"They asked if I had a token code, I said no, they said that's fine - just use our one."
The security of government systems was put under scrutiny last year when it was discovered that data on more than five million people was stolen from the Office of Personnel Management.
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US authorities have acknowledged a data breach affecting the Department of Justice and Department of Homeland Security - but downplayed its severity.