John McCain

"I obviously have seen no evidence of illegal voting," McCain told CNN's Anderson Cooper. "My focus has been on national security, getting Gen. Mattis in as secretary of defense ... doing those issues."

McCain added that he had learned to disregard at least some of the President's newsmaking remarks and focus on the business of governing.

Mr McCain said Mr Trump did not have an "unfettered licence to defame those who are the best among us".

Democratic lawmakers and the soldier's father have called on Republicans to disavow Mr Trump.

US Army Capt Humayun Khan was killed by a car bomb in 2004 in Iraq at the age of 27.

His father, Pakistani-born Khizr Khan, told the BBC on Monday that Mr Trump could not insult women, judges and even members of his own party and not expect to face criticism.

"We all have same equal rights," Mr Khan said.

McCain, a Republican, stood next to an enlarged, close-up photo of Aylan Kurdi, who drowned along with his 5-year-old brother and mother when their small rubber boat capsized as it headed for Greece. 

McCain said the photo has "opened the world's eye to this devastating crisis."

McCain said it should haunt Americans that "the United States will continue to do nothing meaningful" about conflicts like that in Syria that have led to a surge in refugees.

Senate Minority Leader Harry Reid jumped on both Tuesday, first attacking Trump for his criticism of Sen. John McCain, R-Ariz., then pivoting to a larger target: the growing GOP presidential field and the entire Republican Party.

Reid, D-Nev., noted that while Trump's GOP White House rivals were nearly unanimous in denouncing Trump's suggestion that McCain is not really a war hero, they were more tentative in responding to his criticisms of Mexican immigrants as "criminals" and "rapists."

But Trump simply may not care; indeed he seemed to bask in his McCain takedown.

After dismissing McCain's reputation as a war hero because he was captured in Vietnam and "I like people who weren't captured," Trump declared "I will say what I want to say." He insisted he would stay in the GOP primary field, despite rivals who say he's now shown he doesn't merit the presidency.

In Iowa on Saturday, Trump said that he doesn't consider McCain, a prisoner of war in the Vietnam War, to be a war hero.

The front-runner for the Democratic nomination, Hillary Rodham Clinton, not only harshly criticized Trump during a campaign event in Arkansas, she accused the GOP field of being slow to repudiate Trump's other controversial statements.

The Republican told reporters that Afghan national forces are fighting bravely, but suffering heavy losses in the field.

American and international troops have already stopped playing a combat role, remaining as trainers for local forces. The international numbers will be reduced further at the end of 2016. But McCain said reductions should be based on conditions on the ground.