Republican senator John McCain and President Barack Obama have united in their disgust of Donald Trump for his attack on the parents of a Muslim US Army officer killed in the Iraq war.

Decorated war veteran, Mr McCain waded into the dispute between his party’s controversial nominee and Khizr and Ghazala Khan, issuing the strongest rebuke yet to Trump from a senior Republican on the issue.

“While our Party has bestowed upon him the nomination, it is not accompanied by unfettered license to defame those who are the best among us,” wrote McCain, who was a prisoner of war for five years during the Vietnam War.

Trump’s dispute with the Khans has dominated the election campaign in recent days after Mr Khan spoke at the Democratic National Convention on Thursday night, with his wife standing at his side.

Their son, Army Captain Humayun Khan, was killed by a bomb in Iraq in 2004, and the father spoke emotionally of the sacrifice his son had made for the country as an American Muslim, specifically criticising Trump’s proposal to ban Muslims from entering the country.

The parents said Trump is ignorant about Islam and about their family’s sacrifice.

“The nomination is not accompanied by unfettered license to defame those who are the best among us,” John McCain. Photo: Getty

Several leading Republicans have weighed in to express support for the family.

Trump’s off-the-cuff insults and controversial proposals such as the ban on Muslims and a plan to keep illegal immigrants out by building a wall along the Mexican border, have made many in the party establishment reluctant backers of his White House bid.

In his statement, McCain recalled how Humayun Khan died, saying that when a suicide bomber aimed his vehicle toward a building housing hundreds of US soldiers, the captain told his subordinates to stay away, then ran toward it.

McCain thanked the Khans for coming to America, saying: “Your son was the best of America, and the memory of his sacrifice will make us a better nation — and he will never be forgotten.”

In remarks to television networks on Monday, Mr Khan said Trump lacked the empathy to be a leader and chided him for throwing the first salvo in their exchange.

Trump responded simultaneously on Twitter to the parents’ morning television appearances, saying he was being “viciously” attacked.

Trump has tried to shift focus from the Khans.

This story is not about Mr. Khan, who is all over the place doing interviews, but rather RADICAL ISLAMIC TERRORISM and the U.S. Get smart!

— Donald J. Trump (@realDonaldTrump) August 1, 2016

Families of dead soldiers must be honoured: Obama

Speaking at a Disabled American Veterans conference in Georgia, Mr Obama stood firmly agains “the trash-talking” of US military and disrespecting the families of fallen soldiers.

“As commander-in-chief, I’m pretty tired of some folks trash-talking America’s military and troops,” he said.

“Our Gold Star families have made a sacrifice that most of us cannot even begin to imagine.

“We have to do everything we can for those families and honour them. And be humbled by them.”