Media playback is unsupported on your device Media caption Obama: Donald Trump "unfit" to be president

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

The squeeze will continue: analysis by Anthony Zurcher, BBC News North America Reporter

Barack Obama has been steadily upping the pressure on Donald Trump's Republican Party supporters. Last week, at the Democratic National Convention, the president said he didn't think the nominee was within the mainstream of modern conservativism.

On Tuesday he called into question Mr Trump's fitness to hold the presidency - his sharpest words so far - and questioned why the party leaders continue to stand by their man.

Those comments take dead aim at Speaker of the House Paul Ryan, Senate Majority Leader Mitch McConnell and 2008 presidential nominee John McCain, who all have issued statements distancing themselves from Mr Trump's recent feud with the parents of a Muslim-American soldier killed in Iraq.

The irony, of course, is that Mr Obama's remarks likely make it more difficult for the party's top men to renounce their standard-bearer, lest they appear to be caving to their Democratic nemesis. Instead, the squeeze will continue - and grow more politically intolerable with each new Trump controversy.

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.

Image copyright AFP Image caption Khizr Khan spoke out against Mr Trump's policies at the Democratic National Convention

"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: