Barack Obama to launch campaign blitz for Hillary Clinton

US President Barack Obama will launch a campaign blitz for Democrat Hillary Clinton next week as the race against Republican Donald Trump for the White House enters the final stretch.

The Clinton camp announced on Friday that Obama will make the case for his former secretary of state for three straight days in as many key battleground states.

Obama's appearances could be crucial after the Federal Bureau of Investigation said it was reviewing newly discovered emails that "appear to be pertinent" to the investigation into Clinton's use of private email while at the State Department.

On Tuesday, Obama will stump for her in Columbus, Ohio. The president will campaign in Raleigh, North Carolina, on Wednesday, followed by swings in southern Florida, including the Jacksonville area, on Thursday.

The three states account for 62 of the electoral college votes needed to reach the 270-vote minimum to win the White House in the November 8 election.

Obama won the states in 2008 and 2012, except in the case of North Carolina, which he won the first time but lost in his re-election bid.

The Clinton team has intensely targeted voter campaigns in those states, where a defeat of Trump would virtually ensure his loss in the election.

Clinton is also scheduled to campaign in Ohio, North Carolina and Florida next week, but has added an unusual stop to her calendar Arizona, a Republican Party stronghold where she is leading in several public opinion polls.

"Arizona is another battleground state that is now on the map," campaign manager Robby Mook told reporters aboard her plane as the candidate flew into Iowa, another swing state that had been leaning toward Trump but where the polls have tightened into nearly a dead heat.

The southwestern state of Arizona, home to the 2008 Republican nominee John McCain, is "a contest that I think we can win and that's why we want to spend some time there," Mook said.

"But it's a toss-up, and so are Ohio, Iowa.