After drawing criticisms for his comments about Mexican immigrants, Trump lets off another diatribe questioning whether US Senator John McCain is a war hero - and he is not apologizing for it.

Trump, a Republican presidential contender, said at a gathering of religious conservatives Saturday in Iowa that McCain is only a war hero because he was captured.

The comment came after the event moderator, Frank Luntz, referenced the Arizona senator as a war hero during a question-and-answer session.

"He is not a war hero," Trump told Luntz. "I like people that weren't captured, OK? I hate to tell you. He is a war hero because he was captured."

The comment drew both boos and cheers from the audience.

Later, at a news conference, the real estate billionaire tried to soften the blow. He said, "if a person is captured, they are a hero as far as I'm concerned."

In an interview with ABC News on Sunday, Trump said he owed McCain no apology.

"People that fought hard and weren't captured and went through a lot, they get no credit. Nobody even talks about them. They're like forgotten."

Trump's remarks against McCain, a Navy fighter pilot, drew rebuke from rival Republican presidential contenders.

Air Force veteran and former Texas Gov. Rick Perry, who was also at the Iowa gathering remarked, "I tell people all the time it's OK to question your government. But don't question the men and women of the military who sacrifice and sometimes pay a huge price for our safety and our freedom and our economics."

Florida Gov. Jeb Bush tweeted, "enough with the slanderous attacks. @SenJohnMcCain and all our veterans - particularly POWs have earned our respect and admiration."

McCain also found an ally in 2012 presidential candidate, Mitt Romney, on Twitter. "The difference between @SenJohnMcCain and @realDonaldTrump: Trump shot himself down. McCain and American veterans are true heroes."

Concerned about the impact of Trump's comments about illegal immigration on the GOP brand, the Republican National Committee also denounced the businessman's recent remarks.

Sean Spicer, the committee's chief strategist said, "There is no place in our party or our country for comments that disparage those who have served honorably."

Hillary Clinton, a presidential candidate for the Democratic Party, saw an opening to hit Trump.

"There's nothing funny about the hate he is spewing at immigrants and their families, and now the insults he's directed at a genuine war hero", Clinton rebuked.

 "It's shameful, and so is the fact that it took so long for most of his fellow Republican candidates to start standing up to him," Clinton told an audience during a democratic dinner in Arkansas.

McCain spent five years as a prisoner after being shot down in 1967, during the Vietnam War. He suffered torture at the hands of his captors, limiting the mobility of his arms.