Hillary Clinton won't face charges over email scandal

US presidential hopeful Hillary Clinton can now breathe a sigh of relief after FBI Director James Comey's announcement on Tuesday that the agency will not recommend she face charges over her use of a personal email server while she was secretary of state.

Clinton's campaign welcomed the end of a probe that has cast a cloud over her campaign, but Republicans seized on Comey's criticism of what he termed Clinton's "extremely careless" handling of emails.

Clinton's campaign was anxious to move on after Comey's announcement, saying in a statement it was "pleased" with the FBI decision.

"As the secretary has long said, it was a mistake to use her personal email and she would not do it again. We are glad that this matter is now resolved," spokesman Brian Fallon said.

Presumptive Republican nominee Donald Trump, who was also due to campaign in North Carolina on Tuesday, said of the FBI recommendation, "As usual, bad judgment."

Earnest said on Tuesday that Obama did not get advance notice of Comey's announcement and that he would not discuss the FBI investigation with Clinton.

Obama Joins Clinton On Campaign Trail

US Pesident Barack Obama and Clinton arrived in Charlotte, North Carolina, in what is likely to be the first of many trips to urge voters to pick his onetime rival as his successor.

In Clinton's June 2008 concession speech for the Democratic US presidential nomination, she pledged to do all she could to propel Obama to the White House.

On Tuesday, he returned the favour.

Obama endorsed Clinton last month with a forceful video in which he stated that no one had been so qualified for the job.

But a joint appearance planned for soon after that was postponed after the mass shooting on June 12 at a gay nightclub in Orlando, Florida.

"You will hear from him at length today about why he is so enthusiastic about her candidacy and why he thinks she would be an excellent 45th president of the United States," White House spokesman Josh Earnest told reporters before the event on Tuesday.

Clinton hopes to reclaim North Carolina for Democrats in the November 8 election.

Obama won the state in the 2008 general election, but lost it narrowly in 2012.

Obama's first campaign appearance with the former first lady will close a circle on a relationship that started cordially when the two were colleagues in the US Senate, grew tense when they were rivals in the 2008 race, and became close when Clinton was secretary of state during Obama's first term.

Obama has focused on what he touts as Clinton's strength of character, in hopes of shoring up support among voters who find her untrustworthy, a weakness Trump has sought to exploit.

She and Obama traveled to North Carolina on the presidential plane Air Force One, a move Trump said was a burden on taxpayers.

A Clinton spokesman said the campaign would cover its portion of the travel costs.

Clinton needs Obama to woo young and left-leaning voters who backed Sanders and who made up part of the president's voting coalition in 2008 and 2012.

Clinton has also campaigned with high-profile liberal US Senator Elizabeth Warren, and she will appear later this week with Vice President Joe Biden.