Obama 'guarantees' Clinton email probe's independence

US President Barack Obama said Sunday that the FBI investigation into Hillary Clinton's use of a private email server during her time as secretary of state is taking place without political interference.

The controversy over the Democratic presidential frontrunner's management of her official correspondence as US top diplomat from 2009 to 2013 has provided a staple Republican line of attack during the campaign for the White House.

"I guarantee that there is no political influence in any investigation conducted by the Justice Department or the FBI, not just in this case but in any case. Period," Obama told Fox News Sunday.

Presidential candidate Hillary Clinton, who is leading the Democratic campaign against Vermont Senator Bernie Sanders, has said that none of the emails sent over her private server were classified at the time ©Kena Betancur (AFP/File)

"I do not talk to the attorney general about pending investigations," he added. "I do not talk to FBI directors about pending investigations."

Asked by journalist Chris Wallace whether Clinton would be treated differently were she to win the Democratic nomination, Obama said, "How many times do I have to say it, Chris? Guaranteed."

Obama, who leaves office in January, restated his view that although Clinton in her own words had shown a degree of "carelessness" over her official emails at the State Department, "she has not jeopardized America's national security."

Calling on the former first lady's critics to "keep this in perspective," he said she had done "an outstanding job" during her four years as secretary of state.

Clinton, who is leading the Democratic campaign against Vermont Senator Bernie Sanders, has said that none of the emails sent over her private server were classified at the time.

She has handed the State Department 52,000 pages of documents relating to her correspondence.