'Worst president in US history'

Donald Trump has dismissed Barack Obama's time in the White House as a "disaster" after the US president said he was not fit to succeed him. Obama also questioned why Trump's party hasn't disowned him.

"He's been weak, he's been ineffective," Republican candidate Trump said of Obama in a Fox News interview on Tuesday.

Trump has also turned on two senior figures in his own party who have publicly criticised him.

In an interview for the Washington Post, he refused to endorse House Speaker Paul Ryan and Senator John McCain, who are up for re-election in November.

Amid the feuding within Republican ranks, prominent party donor and fundraiser Meg Whitman has publicly endorsed Democrat Hillary Clinton, saying Donald Trump's "demagoguery" had undermined the national fabric.

"To vote Republican out of party loyalty alone would be to endorse a candidacy that I believe has exploited anger, grievance, xenophobia and racial division," she wrote on Facebook.

A Reuters/Ipsos opinion poll suggested Hillary Clinton had extended her lead over Trump to eight percentage points, from six points on Friday.

French President Francois Hollande joined the chorus of criticism on Tuesday, saying that Trump made people "feel nauseous".

"Should the American people choose Trump, there will be consequences, because a US election is a global election," he added.

Speaking to Fox, Trump said Obama had been "the worst president, maybe, in the history of our country".

Obama and Clinton, his one-time secretary of state, had "destabilised the Middle East" while putting the "country at risk" with Clinton's use of a private email server, he said.

Trump is under fire for attacking the parents of a dead US Muslim soldier after they criticised him at the Democratic convention last week.

"The Republican nominee is unfit to serve as president and he keeps on proving it," Obama said on Tuesday.

"The notion that he would attack a Gold Star family that made such extraordinary sacrifices... means that he is woefully unprepared to do this job."

New York Representative Richard Hanna became the first Republican member of Congress to publicly say he would vote for Clinton.

Until recently, many Republicans opposed to Trump had stopped short of supporting Clinton, saying they would vote for a third party or "write-in" candidate.