CEDAR RAPIDS, US -  The FBI dealt Hillary Clinton's seemingly unstoppable White House campaign a stunning blow Friday by reopening a probe into her use of a private email server while secretary of state.

With just 11 days to go until America votes, Clinton's rival Donald Trump seized triumphantly on the shock decision, declaring the Democrat unfit for office in front of a crowd chanting: "Lock her up!"

Clinton was on a flight when the news broke and ignored reporters' shouted questions about the probe as she landed - smiling and waving - in Cedar Rapids, Iowa, for campaign events of her own.

Clinton's vice-presidential running mate Tim Kaine, appearing in Tallahassee, Florida, appeared to be caught off guard by the decision, telling reporters: "I've got to read more. I've got to read a little more."

FBI director James Comey dropped the bombshell in a letter to congressional committees investigating allegations that Clinton put US secrets at risk during her time as secretary of state.

Comey had previously told lawmakers the FBI probe into Clinton's unusual decision to use a private server instead of a government email while in office had concluded without finding wrongdoing.

But, writing to "supplement" this assessment, he said that "in connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation."

The FBI will thus take "appropriate investigative steps" to decide whether a new batch of mails contains classified information "as well as to assess their importance to the investigation."

The 70-year-old Republican nominee Trump, trailing in opinion polls both nationally and in the key swing states he must win to secure the White House, seized triumphantly on the news.

"We must not let her take her criminal scheme into the Oval Office," he told cheering crowds at a campaign rally in Manchester, New Hampshire.

"I have great respect for the fact that the FBI and the Department of Justice are now willing to have the courage to right the horrible mistake that they made," he added.

The news took the shine off what should have been a good day for Clinton, with President Barack Obama's Democratic administration announcing stronger than expected economic growth numbers.

While the 69-year-old former first lady holds an overall poll lead and remains on course to become America's first female president, a handful of states - including Iowa - could be close-fought.

Both candidates were to be in the battleground of Iowa within hours of each other as they vie to drum up support across the rolling plains of the Midwest.

From there, Clinton was to head to the state capital Des Moines to rally voters.

Trump kicked off the day in New Hampshire, jetting to Maine and was to wrap up with a rally for 5,000 people in an open-air amphitheater in Cedar Rapids.

While Trump draws the biggest crowds, he is failing to pull in donations to match: contributions to his campaign slumped in October.

Both camps filed their campaign accounts Thursday night for the period leading up to October 19. These confirmed the Democrats' overpowering lead in the money game, even though the total sums raised fall short of the records set in 2012.

Meanwhile, 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 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."