Democratic US presidential candidate Hillary Clinton listens to US President Barack Obama address a Clinton campaign event in Charlotte, North Carolina, US, July 5, 2016. [Photo/Agencies]

CHARLOTTE, the United States - US President Barack Obama on Tuesday showered praise on Hillary Clinton's judgement and toughness hours after FBI Director James Comey blasted the presumptive Democratic presidential nominee for being "extremely careless" in handling classified information.

"I'm here today because I believe in Hillary Clinton," said Obama during his 2016 debut on the campaign trail at a rally in Charlotte, North Carolina. "There has never been a man or woman more qualified for this office than Hillary Clinton. Ever. And that's the truth."

Speaking to a roaring crowd of Clinton supporters, Obama reminded them and TV viewers across the country of his bitter experience of competing against the strong-minded former first lady in the 2008 primary season and of Clinton's "grace and the energy with which she threw herself into my campaign."

"I came away from that primary admiring her even more, because during that year and a half, I had a chance to see up close just how smart she was and just how prepared she was," said Obama who hailed Hillary in first joint campaign appearance.

While slamming Donald Trump, the presumptive Republican nominee, for his "tough talk" and "phony bluster", Obama lauded Clinton's past record as his top diplomat as evidence of her exceptional judgement and toughness.

"My faith in Hillary Clinton has always been rewarded. I have had a front-row seat to her judgement and her toughness and her commitment to diplomacy. And I witnessed it in the Situation Room where she argued in favor of the mission to get Bin Laden," said Obama.

Obama's praise of Clinton's judgement came just hours after FBI chief Comey criticized her for her private email setup during her stint at the State Department, describing her and her senior aides as being "extremely careless" in handling sensitive information.

Comey said federal investigators found that 110 emails in 52 email chains had contained classified information at the time they were sent or received and that eight of those 52 chains "contained information that was top secret" when they were sent.