Democratic presidential nominee Hillary Clinton has extended her lead over Republican rival Donald Trump to eight percentage points, according to a Reuters/Ipsos poll.

The bump for Mrs Clinton followed the Democratic National Convention in Philadelphia last week, at which Mrs Clinton and her supporters painted an optimistic portrait of America, and the controversy surrounding Mr Trump's comments about the parents of a slain Muslim US soldier.

About 43 per cent of likely voters favoured Mrs Clinton, 35 per cent favoured Mr Trump, and 9 per cent picked "other", according to the July 28- August 1 online poll of 1,289 likely voters.

The poll had margin of error of 3 percentage points.

Mr Trump has faced a barrage of criticism in recent days from political allies as well as opponents after he criticised the parents of US Army Captain Humayun Khan, who was killed in Iraq in 2004.

Sorry, this video has expired Barack Obama says Donald Trump 'unfit' to serve as US president

His father, Khizr Khan, had harsh words for Mr Trump at the Democratic convention where he spoke last week, saying the 70-year-old businessman and former reality TV star had "sacrificed nothing and no-one" and railed against his calls to ban Muslim immigrants.

Mr Trump responded, saying Mr Khan "viciously attacked" him, and suggested the man was repeating lines fed to him by the Clinton campaign.

New Jersey Governor Chris Christie, who dropped out of the Republican presidential primary race and endorsed Mr Trump, said on Tuesday (local time) Mr Trump's criticisms of the Khans were "inappropriate".

Republican congressman Richard Hanna said it was "not enough to simply denounce his [Mr Trump's] comments," and said he would support Mrs Clinton instead.

Mr Trump had a short-lived bump in the polls after the Republican National Convention in Cleveland in late July, when Reuters polling showed him slightly ahead of Mrs Clinton for the first time since May.

Sorry, this video has expired Parents of slain American Muslim soldier hit back at Donald Trump

Senior Democrat officials resign following latest email leak

Three senior officials with the Democratic National Committee (DNC) have resigned following a hack of thousands of emails.

The leaks embarrassed the party just as it staged its national convention last week.

Those who resigned were the chief executive officer, the communications director and the chief financial officer.

The DNC's announcement of the resignations did not mention the hacks or give a reason, but cast the changes as looking ahead to the US election in November.

Reuters/ABC