US President Barack Obama has called Republican White House nominee Donald Trump "unfit" to be president, and said he "keeps on proving it." Obama cited Trump's criticism of a fallen Muslim-American soldier's family as the latest evidence. CCTV's Nathan King has this report.

Even in the U.S. it is rare for a sitting president to call a rival candidate unfit for office but Barack Obama is doing just that-calling on Republicans to dump their candidate. 

"There has to come a point at which you say somebody who makes those kinds of statements doesn't have the judgement, the temperament, the understanding to occupy the most powerful position in the world," said US president Barack Obama.

It was meant to be very different-the Republican nominee Donald Trump was supposed to come out of the convention focusing on his rival Hillary Clinton and victory in the November general election.

But this stirring speech by the father of a fallen Muslim American soldier has sidetracked the Trump campaign.

Khizr Khan, a father of muslim-american soldier, said, "You have sacrificed nothing and no one."

Trump responded on social media saying "I was viciously attacked by Mr. Khan at the Democratic Convention. Am I not allowed to respond Hillary voted for the Iraq war, not me!"

But Mr. Khan's emotional story has resonated across the political spectrum. In an interview on CCTV he movingly talks of his son and his sacrifice.

"If he was with us, he'd be standing right here. Because that's what he was. His hand would be on my shoulder." Khizr Khan said.

Trump does say Mr. Khan's son was a hero, but Trump stood by his proposals to ban Muslim immigration as a way to stop potential terrorists entering the U.S.

But he then seemed to equate his own sacrifices in the business world with the Khan's sacrifice of their son. And now, there are questions about how Trump avoided military service in the Vietnam war.

The damage has been done. Many leaders in Trump's own party have criticized his comments and veterans groups - a core Republican constituency - have criticized Trump, too. Trump had been counting on the military vote to help win the White House in November.

Trump's rival Hillary Clinton says his criticism of a family who gave their son in defense of the country sunk to a new low in an already nasty election campaign. Trump hit back, highlighting Clinton's support for the Iraq war in which the Khan's son died.

"Trump's support, however, seems to be sagging. In the latest polls Hillary Clinton seems to be building a stronger lead. While traditional Trump supporters praise his plain speaking, his salvoes against what Americans call a Gold Star family - a family that lost a child in the service of the country - has offended many. It could cost him votes in an election now less than 100 days away. " Nathan King said from Washington.