WASHINGTON (REUTERS) - An image purported to be a scanned copy of US First Lady Michelle Obama's passport was leaked online on Thursday (Sept 22) alongside personal emails said to belong to a low-level White House staffer who worked with Hillary Clinton's presidential campaign.

Reuters was unable to verify the authenticity of the passport or related documents, the latest dump of sensitive material by a hacking entity US intelligence officials suspect is linked to Russia.

The White House on Thursday declined to comment on their validity, but spokesman Josh Earnest said the administration was taking the matter seriously.

"We're aware of those media reports, and it is something we're looking into," US Attorney-General Loretta Lynch said at a news conference.

The e-mails published on Thursday appear to be from a Gmail account belonging to staffer Ian Mellul and largely contain mundane information concerning planning logistics for Clinton campaign events.

The leak is the latest in a set of files targeting US politicians and political insiders to be disclosed by a group calling itself DC Leaks.

Last week, the group published personal e-mails from former Secretary of State Colin Powell showing his distaste for Clinton and her Republican rival, Donald Trump.

Powell confirmed to Reuters the hacked messages were authentic.

Cyber security experts and US intelligence officials have said the DC Leaks group, which says it is operating in the name of anti-secrecy, is a front for a wide-ranging hacking operation by the Russian government that also has breached Democratic party organisations and at least two state election systems.

The US Secret Service "is concerned any time unauthorised information that might pertain to one of the individuals we protect, or our operations, is allegedly disclosed," said Nicole Mainor, a spokeswoman for the agency.

She declined to comment further, citing a policy of not providing information about investigations.