President Barack Obama has said Republican nominee Donald Trump is unfit to be president, and questioned why his party still supports the New York billionaire's candidacy.

 "There has to come a point at which you say: 'Enough'," Mr Obama said.

Mr Trump is under fire for attacking the parents of a dead US Muslim soldier after they criticised him.

 On Tuesday, he 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.

Mr Trump, who faces the Democrats' Hillary Clinton in November's presidential election, has also been condemned for backing the Russian annexation of Crimea.

Mr Obama said he had had policy differences with previous Republican presidents and candidates - but added that he had never thought they could not function as president.

 In other developments:

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

    A federal judge who has been a target of Mr Trump's repeated scorn denied a media request to release videos of the candidate testifying in a lawsuit about the now-defunct Trump University; Mr Trump's lawyers had argued the videos would have been used to tarnish his campaign

At last week's Democratic National Convention, Khizr Khan - a Muslim whose son was killed serving in the US military in Iraq - criticised Mr Trump's plan to temporarily ban Muslims from entering the US.

 Mr Trump responded by attacking the "Gold Star" family, the term for families that have lost a close relative in war. Democratic and Republican leaders as well as veterans' groups quickly condemned him.

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

 Mr Trump released a statement on Tuesday afternoon responding to "President Obama's failed leadership" but did not directly address the president's criticisms.

 "Our nation has been humiliated abroad and compromised by radical Islam brought onto our shores," Mr Trump said.

On Tuesday, New York Representative Richard Hanna became the first Republican member of Congress to publicly say he would vote for Mrs Clinton. Mr Hanna said Mr Trump's comments about the Khan family had been the deciding factor.

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

Also on the campaign trail:

    Mr Trump was handed a Purple Heart - a medal given to wounded members of the US military - at campaign rally in Virginia. "I always wanted to get the Purple Heart," Mr Trump said. "This was much easier." The comments have drawn criticism from veterans' groups online.

    Mr Trump joked that a mother and her crying baby should be ejected from the Virginia rally. "You can get the baby out of here," he said from the stage.

    Several top Democratic National Committee staff members including its CEO have stepped down after embarrassing emails were made public.

    Mr Trump son, Eric, told an interviewer that a "strong, powerful woman" like his sister wouldn't allow sexual harassment to occur. His father said he would advise his daughter to find another job if she were the victim of sexual harassment. Both Trumps were discussing the recent ouster of Fox News boss Roger Ailes after allegations of sexual harassment.